diff --git a/src/tools/xml_tools.py b/src/tools/xml_tools.py
index 659148a9..9804a70a 100644
--- a/src/tools/xml_tools.py
+++ b/src/tools/xml_tools.py
@@ -334,8 +334,10 @@ class XMLUI(object):
         elem.setAttribute('value', value)
         return elem
 
-    def addList(self, options, name=None, value=None, style=set()):
+    def addList(self, options, name=None, value=None, style=None):
         """Add a list of choices"""
+        if style is None:
+            style = set()
         styles = set(style)
         assert options
         assert styles.issubset(['multi'])