diff --git a/proptest-derive/tests/compile-fail/must-be-debug.rs b/proptest-derive/tests/compile-fail/must-be-debug.rs
index 0a66b0cb..5c1f6db0 100644
--- a/proptest-derive/tests/compile-fail/must-be-debug.rs
+++ b/proptest-derive/tests/compile-fail/must-be-debug.rs
@@ -11,5 +11,5 @@ extern crate proptest_derive;
 
 fn main() {}
 
-#[derive(Arbitrary)]
+#[derive(Arbitrary)] //~ `Foo` doesn't implement `Debug` [E0277]
 struct Foo { x: usize } //~ `Foo` doesn't implement `Debug` [E0277]