diff --git a/src/codec/error.rs b/src/codec/error.rs
index 12105a4..bdd2621 100644
--- a/src/codec/error.rs
+++ b/src/codec/error.rs
@@ -37,6 +37,9 @@ pub enum UserError {
 
     /// The application attempted to initiate too many streams to remote.
     Rejected,
+
+    /// The released capacity is larger than claimed capacity.
+    ReleaseCapacityTooBig,
 }
 
 // ===== impl RecvError =====
@@ -109,6 +112,7 @@ impl error::Error for UserError {
             UnexpectedFrameType => "unexpected frame type",
             PayloadTooBig => "payload too big",
             Rejected => "rejected",
+            ReleaseCapacityTooBig => "release capacity too big",
         }
     }
 }
diff --git a/src/proto/streams/recv.rs b/src/proto/streams/recv.rs
index d0c9772..252a182 100644
--- a/src/proto/streams/recv.rs
+++ b/src/proto/streams/recv.rs
@@ -215,9 +215,9 @@ impl<B, P> Recv<B, P>
         -> Result<(), UserError>
     {
         trace!("release_capacity; size={}", capacity);
+
         if capacity > stream.in_flight_recv_data {
-            // TODO: Handle error
-            unimplemented!();
+            return Err(UserError::ReleaseCapacityTooBig);
         }
 
         // Decrement in-flight data