diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 628bf654873c7..4033fea1f8e5d 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1569,7 +1569,7 @@ impl Debug for () {
     }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Debug for PhantomData<T> {
+impl<T: ?Sized> Debug for PhantomData<T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
         f.pad("PhantomData")
     }