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":
diff --git a/py_gen/tests/of12.py b/py_gen/tests/of12.py
index 3b0aefb..b0dec92 100644
--- a/py_gen/tests/of12.py
+++ b/py_gen/tests/of12.py
@@ -78,7 +78,7 @@
         expected = ''.join([
             '\x80\x00', # class
             '\x02', # type/masked
-            '\x08', # length
+            '\x04', # length
             '\x00\x00\x00\x2a' # value
         ])
         self.assertEquals(expected, obj.pack())
@@ -89,7 +89,7 @@
         expected = ''.join([
             '\x80\x00', # class
             '\x03', # type/masked
-            '\x0c', # length
+            '\x08', # length
             '\x00\x00\x00\x2a', # value
             '\xaa\xbb\xcc\xdd' # mask
         ])
@@ -101,7 +101,7 @@
         expected = ''.join([
             '\x80\x00', # class
             '\x36', # type/masked
-            '\x14', # length
+            '\x10', # length
             '\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0d\x0f', # value
         ])
         self.assertEquals(expected, obj.pack())
diff --git a/py_gen/tests/of13.py b/py_gen/tests/of13.py
index 915bbe8..09ec013 100644
--- a/py_gen/tests/of13.py
+++ b/py_gen/tests/of13.py
@@ -93,7 +93,7 @@
         expected = ''.join([
             '\x80\x00', # class
             '\x02', # type/masked
-            '\x08', # length
+            '\x04', # length
             '\x00\x00\x00\x2a' # value
         ])
         self.assertEquals(expected, obj.pack())
@@ -104,7 +104,7 @@
         expected = ''.join([
             '\x80\x00', # class
             '\x03', # type/masked
-            '\x0c', # length
+            '\x08', # length
             '\x00\x00\x00\x2a', # value
             '\xaa\xbb\xcc\xdd' # mask
         ])
@@ -116,7 +116,7 @@
         expected = ''.join([
             '\x80\x00', # class
             '\x36', # type/masked
-            '\x14', # length
+            '\x10', # length
             '\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0d\x0f', # value
         ])
         self.assertEquals(expected, obj.pack())