diff --git a/mozilla/js/src/jsinterp.c b/mozilla/js/src/jsinterp.c
index cfc55f41788..c9b83d24948 100644
--- a/mozilla/js/src/jsinterp.c
+++ b/mozilla/js/src/jsinterp.c
@@ -1252,6 +1252,7 @@ js_Interpret(JSContext *cx, jsval *result)
 	    if (!ok)
 		goto out;
 	    PUSH_OPND(BOOLEAN_TO_JSVAL(prop != NULL));
+            OBJ_DROP_PROPERTY(cx, obj, prop);
 	    break;
 #endif /* JS_HAS_IN_OPERATOR */