diff --git a/tower-balance/src/error.rs b/tower-balance/src/error.rs
new file mode 100644
index 0000000..9fa7959
--- /dev/null
+++ b/tower-balance/src/error.rs
@@ -0,0 +1,18 @@
+use std::fmt;
+
+pub(crate) type Error = Box<dyn std::error::Error + Send + Sync>;
+
+#[derive(Debug)]
+pub struct Balance(pub(crate) Error);
+
+impl fmt::Display for Balance {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "load balancing discover error: {}", self.0)
+    }
+}
+
+impl std::error::Error for Balance {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        Some(&*self.0)
+    }
+}
diff --git a/tower-balance/src/lib.rs b/tower-balance/src/lib.rs
index cdd4f67..0b40704 100644
--- a/tower-balance/src/lib.rs
+++ b/tower-balance/src/lib.rs
@@ -13,16 +13,18 @@ extern crate tower_service;
 use futures::{Async, Future, Poll};
 use indexmap::IndexMap;
 use rand::{rngs::SmallRng, SeedableRng};
-use std::marker::PhantomData;
-use std::{error, fmt};
+use std::fmt;
 use tower_discover::Discover;
 use tower_service::Service;
 
 pub mod choose;
+pub mod error;
 pub mod load;
 
-pub use choose::Choose;
-pub use load::Load;
+pub use self::choose::Choose;
+pub use self::load::Load;
+
+use self::error::Error;
 
 /// Balances requests across a set of inner services.
 #[derive(Debug)]
@@ -48,14 +50,7 @@ pub struct Balance<D: Discover, C> {
     not_ready: IndexMap<D::Key, D::Service>,
 }
 
-/// Error produced by `Balance`
-#[derive(Debug)]
-pub enum Error<T, U> {
-    Inner(T),
-    Balance(U),
-}
-
-pub struct ResponseFuture<F: Future, E>(F, PhantomData<E>);
+pub struct ResponseFuture<F>(F);
 
 // ===== impl Balance =====
 
@@ -146,15 +141,24 @@ where
     pub fn num_not_ready(&self) -> usize {
         self.not_ready.len()
     }
+}
 
+impl<D, C> Balance<D, C>
+where
+    D: Discover,
+    D::Error: Into<Error>,
+    C: Choose<D::Key, D::Service>,
+{
     /// Polls `discover` for updates, adding new items to `not_ready`.
     ///
     /// Removals may alter the order of either `ready` or `not_ready`.
-    fn update_from_discover<E>(&mut self) -> Result<(), Error<E, D::Error>> {
+    fn update_from_discover(&mut self) -> Result<(), error::Balance> {
         debug!("updating from discover");
         use tower_discover::Change::*;
 
-        while let Async::Ready(change) = self.discover.poll().map_err(Error::Balance)? {
+        while let Async::Ready(change) =
+            self.discover.poll().map_err(|e| error::Balance(e.into()))?
+        {
             match change {
                 Insert(key, mut svc) => {
                     // If the `Insert`ed service is a duplicate of a service already
@@ -184,9 +188,7 @@ where
     ///
     /// When `poll_ready` returns ready, the service is removed from `not_ready` and inserted
     /// into `ready`, potentially altering the order of `ready` and/or `not_ready`.
-    fn promote_to_ready<Request>(
-        &mut self,
-    ) -> Result<(), Error<<D::Service as Service<Request>>::Error, D::Error>>
+    fn promote_to_ready<Request>(&mut self) -> Result<(), <D::Service as Service<Request>>::Error>
     where
         D::Service: Service<Request>,
     {
@@ -205,7 +207,7 @@ where
                     .not_ready
                     .get_index_mut(idx)
                     .expect("invalid not_ready index");;
-                svc.poll_ready().map_err(Error::Inner)?.is_ready()
+                svc.poll_ready()?.is_ready()
             };
             trace!("not_ready[{:?}]: is_ready={:?};", idx, is_ready);
             if is_ready {
@@ -232,7 +234,7 @@ where
     fn poll_ready_index<Request>(
         &mut self,
         idx: usize,
-    ) -> Option<Poll<(), Error<<D::Service as Service<Request>>::Error, D::Error>>>
+    ) -> Option<Poll<(), <D::Service as Service<Request>>::Error>>
     where
         D::Service: Service<Request>,
     {
@@ -240,7 +242,7 @@ where
             None => return None,
             Some((_, svc)) => match svc.poll_ready() {
                 Ok(Async::Ready(())) => return Some(Ok(Async::Ready(()))),
-                Err(e) => return Some(Err(Error::Inner(e))),
+                Err(e) => return Some(Err(e)),
                 Ok(Async::NotReady) => {}
             },
         }
@@ -258,7 +260,7 @@ where
     /// Ensures that .
     fn choose_and_poll_ready<Request>(
         &mut self,
-    ) -> Poll<(), Error<<D::Service as Service<Request>>::Error, D::Error>>
+    ) -> Poll<(), <D::Service as Service<Request>>::Error>
     where
         D::Service: Service<Request>,
     {
@@ -287,15 +289,17 @@ where
     }
 }
 
-impl<D, C, Request> Service<Request> for Balance<D, C>
+impl<D, C, Svc, Request> Service<Request> for Balance<D, C>
 where
-    D: Discover,
-    D::Service: Service<Request>,
-    C: Choose<D::Key, D::Service>,
+    D: Discover<Service = Svc>,
+    D::Error: Into<Error>,
+    Svc: Service<Request>,
+    Svc::Error: Into<Error>,
+    C: Choose<D::Key, Svc>,
 {
-    type Response = <D::Service as Service<Request>>::Response;
-    type Error = Error<<D::Service as Service<Request>>::Error, D::Error>;
-    type Future = ResponseFuture<<D::Service as Service<Request>>::Future, D::Error>;
+    type Response = Svc::Response;
+    type Error = Error;
+    type Future = ResponseFuture<Svc::Future>;
 
     /// Prepares the balancer to process a request.
     ///
@@ -310,15 +314,16 @@ where
         if let Some(idx) = self.dispatched_ready_index.take() {
             // XXX Should we handle per-endpoint errors?
             self.poll_ready_index(idx)
-                .expect("invalid dispatched ready key")?;
+                .expect("invalid dispatched ready key")
+                .map_err(Into::into)?;
         }
 
         // Update `not_ready` and `ready`.
         self.update_from_discover()?;
-        self.promote_to_ready()?;
+        self.promote_to_ready().map_err(Into::into)?;
 
         // Choose the next service to be used by `call`.
-        self.choose_and_poll_ready()
+        self.choose_and_poll_ready().map_err(Into::into)
     }
 
     fn call(&mut self, request: Request) -> Self::Future {
@@ -330,53 +335,22 @@ where
         self.dispatched_ready_index = Some(idx);
 
         let rsp = svc.call(request);
-        ResponseFuture(rsp, PhantomData)
+        ResponseFuture(rsp)
     }
 }
 
 // ===== impl ResponseFuture =====
 
-impl<F: Future, E> Future for ResponseFuture<F, E> {
+impl<F> Future for ResponseFuture<F>
+where
+    F: Future,
+    F::Error: Into<Error>,
+{
     type Item = F::Item;
-    type Error = Error<F::Error, E>;
+    type Error = Error;
 
     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
-        self.0.poll().map_err(Error::Inner)
-    }
-}
-
-// ===== impl Error =====
-
-impl<T, U> fmt::Display for Error<T, U>
-where
-    T: fmt::Display,
-    U: fmt::Display,
-{
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match *self {
-            Error::Inner(ref why) => fmt::Display::fmt(why, f),
-            Error::Balance(ref why) => write!(f, "load balancing failed: {}", why),
-        }
-    }
-}
-
-impl<T, U> error::Error for Error<T, U>
-where
-    T: error::Error,
-    U: error::Error,
-{
-    fn cause(&self) -> Option<&error::Error> {
-        match *self {
-            Error::Inner(ref why) => Some(why),
-            Error::Balance(ref why) => Some(why),
-        }
-    }
-
-    fn description(&self) -> &str {
-        match *self {
-            Error::Inner(_) => "inner service error",
-            Error::Balance(_) => "load balancing failed",
-        }
+        self.0.poll().map_err(Into::into)
     }
 }
 
@@ -398,7 +372,7 @@ mod tests {
     impl Discover for ReluctantDisco {
         type Key = usize;
         type Service = ReluctantService;
-        type Error = ();
+        type Error = Error;
 
         fn poll(&mut self) -> Poll<Change<Self::Key, Self::Service>, Self::Error> {
             let r = self
@@ -413,10 +387,10 @@ mod tests {
 
     impl Service<()> for ReluctantService {
         type Response = ();
-        type Error = ();
-        type Future = future::FutureResult<(), ()>;
+        type Error = Error;
+        type Future = future::FutureResult<Self::Response, Self::Error>;
 
-        fn poll_ready(&mut self) -> Poll<(), ()> {
+        fn poll_ready(&mut self) -> Poll<(), Self::Error> {
             if self.polls_until_ready == 0 {
                 return Ok(Async::Ready(()));
             }
diff --git a/tower-discover/src/lib.rs b/tower-discover/src/lib.rs
index ce7e6f8..d172781 100644
--- a/tower-discover/src/lib.rs
+++ b/tower-discover/src/lib.rs
@@ -12,6 +12,7 @@ extern crate tower_service;
 use futures::{Async, Poll, Stream};
 use tower_service::Service;
 
+use std::fmt;
 use std::hash::Hash;
 use std::iter::{Enumerate, IntoIterator};
 use std::marker::PhantomData;
@@ -51,7 +52,6 @@ where
 {
     inner: Enumerate<T::IntoIter>,
 }
-
 // ===== impl List =====
 
 impl<T, U> List<T>
@@ -74,7 +74,7 @@ where
 {
     type Key = usize;
     type Service = U;
-    type Error = ();
+    type Error = Never;
 
     fn poll(&mut self) -> Poll<Change<Self::Key, Self::Service>, Self::Error> {
         match self.inner.next() {
@@ -129,6 +129,18 @@ where
     }
 }
 
+#[doc(hidden)]
+#[derive(Debug)]
+pub enum Never {}
+
+impl fmt::Display for Never {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        match *self {}
+    }
+}
+
+impl std::error::Error for Never {}
+
 // check that List can be directly over collections
 #[cfg(test)]
 #[allow(dead_code)]