Fixed inputfield definition

Author: syrusakbary

graphql/type/definition.py

@@ -521,14 +521,14 @@ def _define_field_map(self):
         if callable(fields):
             fields = fields()
 
-        if not isinstance(fields, OrderedDict):
-            fields = OrderedDict(sorted(list(fields.items())))
-
         assert isinstance(fields, collections.Mapping) and len(fields) > 0, (
             '{} fields must be a mapping (dict / OrderedDict) with field names as keys or a '
             'function which returns such a mapping.'
         ).format(self)
 
+        if not isinstance(fields, OrderedDict):
+            fields = OrderedDict(sorted(list(fields.items())))
+
         for field_name, field in fields.items():
             assert_valid_name(field_name)