diff --git a/core/classes/mixin/mixin.factor b/core/classes/mixin/mixin.factor
index 3a92d5193c..56e995899b 100755
--- a/core/classes/mixin/mixin.factor
+++ b/core/classes/mixin/mixin.factor
@@ -101,4 +101,4 @@ M: mixin-instance definition drop f ;
 
 M: mixin-instance forget*
     [ class>> ] [ mixin>> ] bi
-    mixin-class? [ remove-mixin-instance ] [ 2drop ] if ;
+    dup mixin-class? [ remove-mixin-instance ] [ 2drop ] if ;