diff --git a/thriftpy2/thrift.py b/thriftpy2/thrift.py
index a409e56c..92cbb8fc 100644
--- a/thriftpy2/thrift.py
+++ b/thriftpy2/thrift.py
@@ -68,20 +68,24 @@ def __init__(self):
             pass
         return __init__
 
-
-    varnames, defaults = zip(*spec)
     if hasattr(cls, 'thrift_spec'):
         args = []
         kwargs = []
+        varnames = []
+        defaults = []
         for spec_element, t_spec in zip(spec, cls.thrift_spec.values()):
+            varnames.append(spec_element[0])
             if t_spec[-1]:
                 args.append(spec_element[0])
             else:
                 kwargs.append(spec_element)
+                defaults.append(spec_element[1])
+        defaults = tuple(defaults) if defaults else None
 
         args.extend(map('{0[0]}={0[1]!r}'.format, kwargs))
         args = ', '.join(args)
     else:
+        varnames, defaults = zip(*spec)
         args = ', '.join(map('{0[0]}={0[1]!r}'.format, spec))
 
     init = "def __init__(self, {}):\n".format(args)