diff --git a/Source/NSGDictionary.m b/Source/NSGDictionary.m
index 614c0c0b5..42992311f 100644
--- a/Source/NSGDictionary.m
+++ b/Source/NSGDictionary.m
@@ -96,7 +96,7 @@ myEqual(NSObject *self, NSObject *other)
  */
 #define	FAST_MAP_HASH(X)	myHash(X.o)
 #define	FAST_MAP_EQUAL(X,Y)	myEqual(X.o,Y.o)
-#define	FAST_MAP_RETAIN_KEY(X)	[(X).o copyWithZone: map->zone]
+#define	FAST_MAP_RETAIN_KEY(X)	[((id)(X).o) copyWithZone: map->zone]
 
 #include	"FastMap.x"