@@ -25,10 +25,13 @@
 #import "OFNotImplementedException.h"
 
 static struct {
 	Class isa;
 } placeholder;
+
+@interface OFMutableSet_placeholder: OFMutableSet
+@end
 
 @implementation OFMutableSet_placeholder
 - init
 {
 	return (id)[[OFMutableSet_hashtable alloc] init];