diff --git a/generator.py b/generator.py
index d3454c4..695fdf4 100755
--- a/generator.py
+++ b/generator.py
@@ -1004,8 +1004,8 @@
         if param.restrictions is not None:
             # join all formats of the file, take mapping from supported_file if available for an entry
             if type(param.restrictions) is _FileFormat:
-                param_format = ','.join([get_supported_file_types(i, supported_file_formats) if
-                                            get_supported_file_types(i, supported_file_formats)
+                param_format = ','.join([get_supported_file_type(i, supported_file_formats) if
+                                            get_supported_file_type(i, supported_file_formats)
                                             else i for i in param.restrictions.formats])
             else:
                 raise InvalidModelException("Expected 'file type' restrictions for input file [%(name)s], "
@@ -1048,8 +1048,9 @@
             if param.restrictions.n_max is not None:
                 param_node.attrib["max"] = str(param.restrictions.n_max)
         elif type(param.restrictions) is _FileFormat:
-            param_node.attrib["format"] = ",".join(
-                get_supported_file_types(param.restrictions.formats, supported_file_formats))
+            param_node.attrib["format"] = ','.join([get_supported_file_type(i, supported_file_formats) if
+                                            get_supported_file_type(i, supported_file_formats)
+                                            else i for i in param.restrictions.formats])
         else:
             raise InvalidModelException("Unrecognized restriction type [%(type)s] for parameter [%(name)s]"
                                         % {"type": type(param.restrictions), "name": param.name})
@@ -1315,6 +1316,14 @@
     return data_node
 
 
+# Get the supported file format for one given format
+def get_supported_file_type(format_name, supported_file_formats):
+    if format_name in supported_file_formats.keys():
+        return supported_file_formats.get(format_name, DataType(format_name, format_name)).galaxy_extension
+    else:
+        return None
+
+
 def get_supported_file_types(formats, supported_file_formats):
     return set([supported_file_formats.get(format_name, DataType(format_name, format_name)).galaxy_extension
                for format_name in formats if format_name in supported_file_formats.keys()])