diff --git a/src/runtime.rs b/src/runtime.rs
index 50f1d132e..500453b96 100644
--- a/src/runtime.rs
+++ b/src/runtime.rs
@@ -40,8 +40,7 @@ pub struct Sel {
 
 /// A marker type to be embedded into other types just so that they cannot be
 /// constructed externally.
-#[repr(C)]
-struct PrivateMarker;
+enum PrivateMarker { }
 
 /// A type that represents an instance variable.
 #[repr(C)]