diff --git a/common/modules/base.jsm b/common/modules/base.jsm
index 5cdae2b1..2b049555 100644
--- a/common/modules/base.jsm
+++ b/common/modules/base.jsm
@@ -427,7 +427,7 @@ function isinstance(targ, src) {
                 return true;
         }
         else if ("isinstance" in targ) {
-            if (targ.isinstance(src[i]))
+            if (targ.isinstance(src[i]) && targ.isinstance !== isinstance)
                 return true;
         }
         else {