--- a/urltree.py	Thu Feb 12 22:56:21 2009 +0200
+++ b/urltree.py	Thu Feb 12 22:59:44 2009 +0200
@@ -628,8 +628,8 @@
                 # otherwise, fail
                 raise URLError("No value given for required argument: %r" % (key, ))
             
-            # already have a value?
-            if key in kwargs :
+            # already have a non-default value?
+            if key in kwargs and key not in default_labels :
                 # append to old value
                 kwargs[key] = type.append(kwargs[key], value)