diff --git a/loxigen.py b/loxigen.py
index ac5d536..43ff2a8 100755
--- a/loxigen.py
+++ b/loxigen.py
@@ -549,7 +549,7 @@
 
             # Extensions
             experimenter = find_experimenter('of', cls)
-            if experimenter:
+            if experimenter and ofclass.superclass in ['of_bsn_header', 'of_nicira_header']:
                 val = find_type_value(ofclass, 'subtype')
                 type_maps.extension_message_subtype[wire_version][experimenter][cls] = val