--- a/plcopen/structures.py	Wed May 15 00:30:12 2013 +0200
+++ b/plcopen/structures.py	Wed May 15 08:20:17 2013 +0200
@@ -98,7 +98,7 @@
             if one_input_connected:
                 for i, variable in enumerate(output_variables):
                     parameter = variable.getformalParameter()
-                    if not inout_variables.has_key(parameter) and parameter in output_names + ["ENO"]:
+                    if not inout_variables.has_key(parameter) and parameter in output_names + ["", "ENO"]:
                         if variable.getformalParameter() == "":
                             variable_name = "%s%d"%(type, block.getlocalId())
                         else: