--- a/base/univention-system-setup/umc/python/setup/util.py	
+++ a/base/univention-system-setup/umc/python/setup/util.py	
@@ -876,6 +876,7 @@ def is_windowsdomainname(domainname):
 
 
 def domain2windowdomain(domainname):
+	windomain = ''
 	if '.' in domainname:
 		windomain = domainname.split('.')[0]
 	windomain = windomain.upper()