diff --git a/galaxy/converter.py b/galaxy/converter.py
index 7f3e542..7dfd1cb 100755
--- a/galaxy/converter.py
+++ b/galaxy/converter.py
@@ -510,7 +510,7 @@
         rep_node.attrib["min"] = "0"
     # for the ITEMLISTs which have LISTITEM children we only
     # need one parameter as it is given as a string
-    if param.default is not None: 
+    if param.default is not None and param.default is not _Null:  
         rep_node.attrib["max"] = "1"
     rep_node.attrib["title"] = get_galaxy_parameter_name(param)