diff --git a/library/core/src/ops/unsize.rs b/library/core/src/ops/unsize.rs
index f0781ee01fd53..cbafa6d660bef 100644
--- a/library/core/src/ops/unsize.rs
+++ b/library/core/src/ops/unsize.rs
@@ -39,7 +39,10 @@ pub trait CoerceUnsized<T: PointeeSized> {
 
 // &mut T -> &mut U
 #[unstable(feature = "coerce_unsized", issue = "18598")]
-impl<'a, T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<&'a mut U> for &'a mut T {}
+impl<'a, 'b: 'a, T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<&'a mut U>
+    for &'b mut T
+{
+}
 // &mut T -> &U
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<'a, 'b: 'a, T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<&'a U> for &'b mut T {}