diff --git a/pykat/param.py b/pykat/param.py
index 72e9561d01888b157512d1c0f6851172aff8a03d..6644407a2a00d859ef9b6e9d9ad52257b88b2cc3 100644
--- a/pykat/param.py
+++ b/pykat/param.py
@@ -86,7 +86,7 @@ class putter(object):
         self.__owner = weakref.ref(newOwner)
         
     def clearPuts(self):
-        for _ in self.putees:
+        for _ in self.putees.copy():
             _.put(None)
     
     def register(self, toput):