diff --git a/tower-buffer/tests/buffer.rs b/tower-buffer/tests/buffer.rs
index 12945c6..5bd5348 100644
--- a/tower-buffer/tests/buffer.rs
+++ b/tower-buffer/tests/buffer.rs
@@ -7,7 +7,6 @@ use futures::prelude::*;
 use tower_buffer::*;
 use tower_service::*;
 
-use std::fmt;
 use std::thread;
 
 #[test]
@@ -90,7 +89,7 @@ fn when_inner_fails() {
 
     // Make the service NotReady
     handle.allow(0);
-    handle.error(Error("foobar"));
+    handle.error("foobar");
 
     let mut res1 = service.call("hello");
 
@@ -101,39 +100,17 @@ fn when_inner_fails() {
         if let Some(e) = e.downcast_ref::<error::ServiceError>() {
             assert!(format!("{}", e).contains("poll_ready"));
 
-            let e = e
-                .source()
-                .expect("nope 1")
-                .downcast_ref::<tower_mock::Error<Error>>()
-                .expect("nope 1_2");
+            let e = e.source().unwrap();
 
-            match e {
-                tower_mock::Error::Other(e) => assert_eq!(e.0, "foobar"),
-                _ => panic!("unexpected mock error"),
-            }
+            assert_eq!(e.to_string(), "foobar");
         } else {
             panic!("unexpected error type: {:?}", e);
         }
     });
 }
 
-#[derive(Debug)]
-struct Error(&'static str);
-
-impl fmt::Display for Error {
-    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        self.0.fmt(fmt)
-    }
-}
-
-impl ::std::error::Error for Error {
-    fn source(&self) -> Option<&(::std::error::Error + 'static)> {
-        None
-    }
-}
-
-type Mock = tower_mock::Mock<&'static str, &'static str, Error>;
-type Handle = tower_mock::Handle<&'static str, &'static str, Error>;
+type Mock = tower_mock::Mock<&'static str, &'static str>;
+type Handle = tower_mock::Handle<&'static str, &'static str>;
 
 struct Exec;
 
diff --git a/tower-filter/tests/filter.rs b/tower-filter/tests/filter.rs
index a599487..a1cdb2d 100644
--- a/tower-filter/tests/filter.rs
+++ b/tower-filter/tests/filter.rs
@@ -117,8 +117,8 @@ fn saturate() {
     th2.join().unwrap();
 }
 
-type Mock = tower_mock::Mock<String, String, ()>;
-type Handle = tower_mock::Handle<String, String, ()>;
+type Mock = tower_mock::Mock<String, String>;
+type Handle = tower_mock::Handle<String, String>;
 
 fn new_service<F, U>(max: usize, f: F) -> (Filter<Mock, F>, Handle)
 where
diff --git a/tower-in-flight-limit/tests/in_flight_limit.rs b/tower-in-flight-limit/tests/in_flight_limit.rs
index 0bc3d50..312b4d5 100644
--- a/tower-in-flight-limit/tests/in_flight_limit.rs
+++ b/tower-in-flight-limit/tests/in_flight_limit.rs
@@ -208,7 +208,7 @@ fn response_error_releases_capacity() {
     // s1 sends the request, then s2 is able to get capacity
     let r1 = s1.call("hello");
     let request = handle.next_request().unwrap();
-    request.error(());
+    request.error("boom");
 
     r1.wait().unwrap_err();
 
@@ -244,8 +244,8 @@ fn response_future_drop_releases_capacity() {
     });
 }
 
-type Mock = tower_mock::Mock<&'static str, &'static str, ()>;
-type Handle = tower_mock::Handle<&'static str, &'static str, ()>;
+type Mock = tower_mock::Mock<&'static str, &'static str>;
+type Handle = tower_mock::Handle<&'static str, &'static str>;
 
 fn new_service(max: usize) -> (InFlightLimit<Mock>, Handle) {
     let (service, handle) = Mock::new();
diff --git a/tower-mock/Cargo.toml b/tower-mock/Cargo.toml
index 6a1efc0..beb5df3 100644
--- a/tower-mock/Cargo.toml
+++ b/tower-mock/Cargo.toml
@@ -6,4 +6,5 @@ publish = false
 
 [dependencies]
 futures = "0.1"
+tokio-sync = "0.1.3"
 tower-service = { version = "0.2", path = "../tower-service" }
diff --git a/tower-mock/src/error.rs b/tower-mock/src/error.rs
new file mode 100644
index 0000000..1fb3064
--- /dev/null
+++ b/tower-mock/src/error.rs
@@ -0,0 +1,23 @@
+//! Error types
+
+use std::error;
+use std::fmt;
+
+pub(crate) type Error = Box<error::Error + Send + Sync>;
+
+#[derive(Debug)]
+pub struct Closed(());
+
+impl Closed {
+    pub(crate) fn new() -> Closed {
+        Closed(())
+    }
+}
+
+impl fmt::Display for Closed {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "service closed")
+    }
+}
+
+impl error::Error for Closed {}
diff --git a/tower-mock/src/future.rs b/tower-mock/src/future.rs
new file mode 100644
index 0000000..0061196
--- /dev/null
+++ b/tower-mock/src/future.rs
@@ -0,0 +1,40 @@
+//! Future types
+
+use error::{self, Error};
+use futures::{Async, Future, Poll};
+use tokio_sync::oneshot;
+
+/// Future of the `Mock` response.
+#[derive(Debug)]
+pub struct ResponseFuture<T> {
+    rx: Option<Rx<T>>,
+}
+
+type Rx<T> = oneshot::Receiver<Result<T, Error>>;
+
+impl<T> ResponseFuture<T> {
+    pub(crate) fn new(rx: Rx<T>) -> ResponseFuture<T> {
+        ResponseFuture { rx: Some(rx) }
+    }
+
+    pub(crate) fn closed() -> ResponseFuture<T> {
+        ResponseFuture { rx: None }
+    }
+}
+
+impl<T> Future for ResponseFuture<T> {
+    type Item = T;
+    type Error = Error;
+
+    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+        match self.rx {
+            Some(ref mut rx) => match rx.poll() {
+                Ok(Async::Ready(Ok(v))) => Ok(v.into()),
+                Ok(Async::Ready(Err(e))) => Err(e),
+                Ok(Async::NotReady) => Ok(Async::NotReady),
+                Err(_) => Err(error::Closed::new().into()),
+            },
+            None => Err(error::Closed::new().into()),
+        }
+    }
+}
diff --git a/tower-mock/src/lib.rs b/tower-mock/src/lib.rs
index 6cfe2fe..7e26985 100644
--- a/tower-mock/src/lib.rs
+++ b/tower-mock/src/lib.rs
@@ -1,88 +1,78 @@
 //! Mock `Service` that can be used in tests.
 
 extern crate futures;
+extern crate tokio_sync;
 extern crate tower_service;
 
-use tower_service::Service;
+pub mod error;
+pub mod future;
 
-use futures::sync::{mpsc, oneshot};
+use error::Error;
+use future::ResponseFuture;
 use futures::task::{self, Task};
 use futures::{Async, Future, Poll, Stream};
+use tokio_sync::{mpsc, oneshot};
+use tower_service::Service;
 
 use std::collections::HashMap;
-use std::fmt;
 use std::sync::{Arc, Mutex};
 use std::{ops, u64};
 
 /// A mock service
 #[derive(Debug)]
-pub struct Mock<T, U, E> {
+pub struct Mock<T, U> {
     id: u64,
-    tx: Mutex<Tx<T, U, E>>,
-    state: Arc<Mutex<State<E>>>,
+    tx: Mutex<Tx<T, U>>,
+    state: Arc<Mutex<State>>,
     can_send: bool,
 }
 
 /// Handle to the `Mock`.
 #[derive(Debug)]
-pub struct Handle<T, U, E> {
-    rx: Rx<T, U, E>,
-    state: Arc<Mutex<State<E>>>,
+pub struct Handle<T, U> {
+    rx: Rx<T, U>,
+    state: Arc<Mutex<State>>,
 }
 
 #[derive(Debug)]
-pub struct Request<T, U, E> {
+pub struct Request<T, U> {
     request: T,
-    respond: Respond<U, E>,
+    respond: Respond<U>,
 }
 
 /// Respond to a request received by `Mock`.
 #[derive(Debug)]
-pub struct Respond<T, E> {
-    tx: oneshot::Sender<Result<T, E>>,
-}
-
-/// Future of the `Mock` response.
-#[derive(Debug)]
-pub struct ResponseFuture<T, E> {
-    // Slight abuse of the error enum...
-    rx: Error<oneshot::Receiver<Result<T, E>>>,
-}
-
-/// Enumeration of errors that can be returned by `Mock`.
-#[derive(Debug, PartialEq)]
-pub enum Error<T> {
-    Closed,
-    Other(T),
+pub struct Respond<T> {
+    tx: oneshot::Sender<Result<T, Error>>,
 }
 
 #[derive(Debug)]
-struct State<E> {
-    // Tracks the number of requests that can be sent through
+struct State {
+    /// Tracks the number of requests that can be sent through
     rem: u64,
 
-    // Tasks that are blocked
+    /// Tasks that are blocked
     tasks: HashMap<u64, Task>,
 
-    // Tracks if the `Handle` dropped
+    /// Tracks if the `Handle` dropped
     is_closed: bool,
 
-    // Tracks the ID for the next mock clone
+    /// Tracks the ID for the next mock clone
     next_clone_id: u64,
 
-    // Tracks the next error to yield (if any)
-    err_with: Option<E>,
+    /// Tracks the next error to yield (if any)
+    err_with: Option<Error>,
 }
 
-type Tx<T, U, E> = mpsc::UnboundedSender<Request<T, U, E>>;
-type Rx<T, U, E> = mpsc::UnboundedReceiver<Request<T, U, E>>;
+type Tx<T, U> = mpsc::UnboundedSender<Request<T, U>>;
+type Rx<T, U> = mpsc::UnboundedReceiver<Request<T, U>>;
 
 // ===== impl Mock =====
 
-impl<T, U, E> Mock<T, U, E> {
+impl<T, U> Mock<T, U> {
     /// Create a new `Mock` and `Handle` pair.
-    pub fn new() -> (Self, Handle<T, U, E>) {
-        let (tx, rx) = mpsc::unbounded();
+    pub fn new() -> (Self, Handle<T, U>) {
+        let (tx, rx) = mpsc::unbounded_channel();
         let tx = Mutex::new(tx);
 
         let state = Arc::new(Mutex::new(State::new()));
@@ -100,16 +90,16 @@ impl<T, U, E> Mock<T, U, E> {
     }
 }
 
-impl<T, U, E> Service<T> for Mock<T, U, E> {
+impl<T, U> Service<T> for Mock<T, U> {
     type Response = U;
-    type Error = Error<E>;
-    type Future = ResponseFuture<U, E>;
+    type Error = Error;
+    type Future = ResponseFuture<U>;
 
     fn poll_ready(&mut self) -> Poll<(), Self::Error> {
         let mut state = self.state.lock().unwrap();
 
         if state.is_closed {
-            return Err(Error::Closed);
+            return Err(error::Closed::new().into());
         }
 
         if self.can_send {
@@ -117,7 +107,7 @@ impl<T, U, E> Service<T> for Mock<T, U, E> {
         }
 
         if let Some(e) = state.err_with.take() {
-            return Err(Error::Other(e));
+            return Err(e);
         }
 
         if state.rem > 0 {
@@ -143,7 +133,7 @@ impl<T, U, E> Service<T> for Mock<T, U, E> {
         let mut state = self.state.lock().unwrap();
 
         if state.is_closed {
-            return ResponseFuture { rx: Error::Closed };
+            return ResponseFuture::closed();
         }
 
         if !self.can_send {
@@ -166,21 +156,19 @@ impl<T, U, E> Service<T> for Mock<T, U, E> {
             respond: Respond { tx },
         };
 
-        match self.tx.lock().unwrap().unbounded_send(request) {
+        match self.tx.lock().unwrap().try_send(request) {
             Ok(_) => {}
             Err(_) => {
                 // TODO: Can this be reached
-                return ResponseFuture { rx: Error::Closed };
+                return ResponseFuture::closed();
             }
         }
 
-        ResponseFuture {
-            rx: Error::Other(rx),
-        }
+        ResponseFuture::new(rx)
     }
 }
 
-impl<T, U, E> Clone for Mock<T, U, E> {
+impl<T, U> Clone for Mock<T, U> {
     fn clone(&self) -> Self {
         let id = {
             let mut state = self.state.lock().unwrap();
@@ -202,7 +190,7 @@ impl<T, U, E> Clone for Mock<T, U, E> {
     }
 }
 
-impl<T, U, E> Drop for Mock<T, U, E> {
+impl<T, U> Drop for Mock<T, U> {
     fn drop(&mut self) {
         let mut state = match self.state.lock() {
             Ok(v) => v,
@@ -221,16 +209,16 @@ impl<T, U, E> Drop for Mock<T, U, E> {
 
 // ===== impl Handle =====
 
-impl<T, U, E> Handle<T, U, E> {
+impl<T, U> Handle<T, U> {
     /// Asynchronously gets the next request
-    pub fn poll_request(&mut self) -> Poll<Option<Request<T, U, E>>, ()> {
-        self.rx.poll()
+    pub fn poll_request(&mut self) -> Poll<Option<Request<T, U>>, Error> {
+        self.rx.poll().map_err(Into::into)
     }
 
     /// Synchronously gets the next request.
     ///
     /// This function blocks the current thread until a request is received.
-    pub fn next_request(&mut self) -> Option<Request<T, U, E>> {
+    pub fn next_request(&mut self) -> Option<Request<T, U>> {
         use futures::future::poll_fn;
         poll_fn(|| self.poll_request()).wait().unwrap()
     }
@@ -248,9 +236,9 @@ impl<T, U, E> Handle<T, U, E> {
     }
 
     /// Make the next poll_ method error with the given error.
-    pub fn error(&mut self, e: E) {
+    pub fn error<E: Into<Error>>(&mut self, e: E) {
         let mut state = self.state.lock().unwrap();
-        state.err_with = Some(e);
+        state.err_with = Some(e.into());
 
         for (_, task) in state.tasks.drain() {
             task.notify();
@@ -258,7 +246,7 @@ impl<T, U, E> Handle<T, U, E> {
     }
 }
 
-impl<T, U, E> Drop for Handle<T, U, E> {
+impl<T, U> Drop for Handle<T, U> {
     fn drop(&mut self) {
         let mut state = match self.state.lock() {
             Ok(v) => v,
@@ -281,9 +269,9 @@ impl<T, U, E> Drop for Handle<T, U, E> {
 
 // ===== impl Request =====
 
-impl<T, U, E> Request<T, U, E> {
+impl<T, U> Request<T, U> {
     /// Split the request and respond handle
-    pub fn into_parts(self) -> (T, Respond<U, E>) {
+    pub fn into_parts(self) -> (T, Respond<U>) {
         (self.request, self.respond)
     }
 
@@ -291,12 +279,12 @@ impl<T, U, E> Request<T, U, E> {
         self.respond.respond(response)
     }
 
-    pub fn error(self, err: E) {
+    pub fn error<E: Into<Error>>(self, err: E) {
         self.respond.error(err)
     }
 }
 
-impl<T, U, E> ops::Deref for Request<T, U, E> {
+impl<T, U> ops::Deref for Request<T, U> {
     type Target = T;
 
     fn deref(&self) -> &T {
@@ -306,67 +294,22 @@ impl<T, U, E> ops::Deref for Request<T, U, E> {
 
 // ===== impl Respond =====
 
-impl<T, E> Respond<T, E> {
+impl<T> Respond<T> {
     pub fn respond(self, response: T) {
         // TODO: Should the result be dropped?
         let _ = self.tx.send(Ok(response));
     }
 
-    pub fn error(self, err: E) {
+    pub fn error<E: Into<Error>>(self, err: E) {
         // TODO: Should the result be dropped?
-        let _ = self.tx.send(Err(err));
-    }
-}
-
-// ===== impl ResponseFuture =====
-
-impl<T, E> Future for ResponseFuture<T, E> {
-    type Item = T;
-    type Error = Error<E>;
-
-    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
-        match self.rx {
-            Error::Other(ref mut rx) => match rx.poll() {
-                Ok(Async::Ready(Ok(v))) => Ok(v.into()),
-                Ok(Async::Ready(Err(e))) => Err(Error::Other(e)),
-                Ok(Async::NotReady) => Ok(Async::NotReady),
-                Err(_) => Err(Error::Closed),
-            },
-            Error::Closed => Err(Error::Closed),
-        }
-    }
-}
-
-// ===== impl Error =====
-
-impl<T> fmt::Display for Error<T>
-where
-    T: fmt::Display,
-{
-    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        match self {
-            Error::Closed => write!(fmt, "mock service is closed"),
-            Error::Other(e) => e.fmt(fmt),
-        }
-    }
-}
-
-impl<T> std::error::Error for Error<T>
-where
-    T: std::error::Error + 'static,
-{
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        match self {
-            Error::Closed => None,
-            Error::Other(e) => Some(e),
-        }
+        let _ = self.tx.send(Err(err.into()));
     }
 }
 
 // ===== impl State =====
 
-impl<E> State<E> {
-    fn new() -> State<E> {
+impl State {
+    fn new() -> State {
         State {
             rem: u64::MAX,
             tasks: HashMap::new(),
diff --git a/tower-mock/tests/mock.rs b/tower-mock/tests/mock.rs
index 0bd0b7f..f40eff3 100644
--- a/tower-mock/tests/mock.rs
+++ b/tower-mock/tests/mock.rs
@@ -50,8 +50,8 @@ fn backpressure() {
     mock.call("hello?".into());
 }
 
-type Mock = tower_mock::Mock<String, String, ()>;
-type Handle = tower_mock::Handle<String, String, ()>;
+type Mock = tower_mock::Mock<String, String>;
+type Handle = tower_mock::Handle<String, String>;
 
 fn new_mock() -> (Mock, Handle) {
     Mock::new()
diff --git a/tower-rate-limit/tests/rate_limit.rs b/tower-rate-limit/tests/rate_limit.rs
index 6696011..4982ab6 100644
--- a/tower-rate-limit/tests/rate_limit.rs
+++ b/tower-rate-limit/tests/rate_limit.rs
@@ -54,8 +54,8 @@ fn reaching_capacity() {
     assert_eq!(response.unwrap(), "done");
 }
 
-type Mock = tower_mock::Mock<&'static str, &'static str, ()>;
-type Handle = tower_mock::Handle<&'static str, &'static str, ()>;
+type Mock = tower_mock::Mock<&'static str, &'static str>;
+type Handle = tower_mock::Handle<&'static str, &'static str>;
 
 fn new_service(rate: Rate) -> (RateLimit<Mock>, Handle) {
     let (service, handle) = Mock::new();
diff --git a/tower-retry/tests/retry.rs b/tower-retry/tests/retry.rs
index 60089b5..688ec27 100644
--- a/tower-retry/tests/retry.rs
+++ b/tower-retry/tests/retry.rs
@@ -48,7 +48,7 @@ fn retry_limit() {
     assert_eq!(*req3, "hello");
     req3.error("retry 3");
 
-    assert_eq!(fut.wait().unwrap_err(), tower_mock::Error::Other("retry 3"));
+    assert_eq!(fut.wait().unwrap_err().to_string(), "retry 3");
 }
 
 #[test]
@@ -66,7 +66,7 @@ fn retry_error_inspection() {
     let req2 = handle.next_request().unwrap();
     assert_eq!(*req2, "hello");
     req2.error("reject");
-    assert_eq!(fut.wait().unwrap_err(), tower_mock::Error::Other("reject"));
+    assert_eq!(fut.wait().unwrap_err().to_string(), "reject");
 }
 
 #[test]
@@ -79,7 +79,7 @@ fn retry_cannot_clone_request() {
     assert_eq!(*req1, "hello");
     req1.error("retry 1");
 
-    assert_eq!(fut.wait().unwrap_err(), tower_mock::Error::Other("retry 1"));
+    assert_eq!(fut.wait().unwrap_err().to_string(), "retry 1");
 }
 
 #[test]
@@ -100,9 +100,9 @@ fn success_with_cannot_clone() {
 type Req = &'static str;
 type Res = &'static str;
 type InnerError = &'static str;
-type Error = tower_mock::Error<InnerError>;
-type Mock = tower_mock::Mock<Req, Res, InnerError>;
-type Handle = tower_mock::Handle<Req, Res, InnerError>;
+type Error = Box<::std::error::Error + Send + Sync>;
+type Mock = tower_mock::Mock<Req, Res>;
+type Handle = tower_mock::Handle<Req, Res>;
 
 #[derive(Clone)]
 struct RetryErrors;
@@ -147,7 +147,7 @@ impl Policy<Req, Res, Error> for UnlessErr {
     type Future = future::FutureResult<Self, ()>;
     fn retry(&self, _: &Req, result: Result<&Res, &Error>) -> Option<Self::Future> {
         result.err().and_then(|err| {
-            if err != &tower_mock::Error::Other(self.0) {
+            if err.to_string() != self.0 {
                 Some(future::ok(self.clone()))
             } else {
                 None