diff --git a/tower/src/buffer/layer.rs b/tower/src/buffer/layer.rs
index f76c044..01e7100 100644
--- a/tower/src/buffer/layer.rs
+++ b/tower/src/buffer/layer.rs
@@ -9,7 +9,7 @@ use tower_service::Service;
 /// which means that this layer can only be used on the Tokio runtime.
 ///
 /// See the module documentation for more details.
-pub struct BufferLayer<Request, E2> {
+pub struct BufferLayer<Request, E2 = crate::BoxError> {
     bound: usize,
     _p: PhantomData<fn(Request, E2)>,
 }
diff --git a/tower/src/buffer/service.rs b/tower/src/buffer/service.rs
index 19c29cf..06d4c55 100644
--- a/tower/src/buffer/service.rs
+++ b/tower/src/buffer/service.rs
@@ -13,7 +13,7 @@ use tower_service::Service;
 ///
 /// See the module documentation for more details.
 #[derive(Debug)]
-pub struct Buffer<S, Request, E2>
+pub struct Buffer<S, Request, E2 = crate::BoxError>
 where
     S: Service<Request>,
 {
diff --git a/tower/src/lib.rs b/tower/src/lib.rs
index 8df5d01..3049301 100644
--- a/tower/src/lib.rs
+++ b/tower/src/lib.rs
@@ -81,3 +81,6 @@ pub use tower_service::Service;
 mod sealed {
     pub trait Sealed<T> {}
 }
+
+/// Alias for a type-erased error type.
+pub type BoxError = Box<dyn std::error::Error + Send + Sync>;