diff --git a/py_gen/codegen.py b/py_gen/codegen.py
index f55610b..90ddf23 100644
--- a/py_gen/codegen.py
+++ b/py_gen/codegen.py
@@ -84,7 +84,7 @@
         oxm_class = 0x8000
         oxm_type = util.primary_wire_type(cls, version)
         oxm_masked = cls.find('masked') != -1 and 1 or 0
-        oxm_len = of_g.base_length[(cls, version)]
+        oxm_len = of_g.base_length[(cls, version)] - 4
         type_values['type_len'] = '%#x' % (oxm_class << 16 | oxm_type << 8 | \
                                            oxm_masked << 8 | oxm_len)
     elif cls == "of_match_v3":