diff --git a/c_gen/build_of_g.py b/c_gen/build_of_g.py
index 047b254..8a7317a 100755
--- a/c_gen/build_of_g.py
+++ b/c_gen/build_of_g.py
@@ -163,6 +163,11 @@
             # but is variable length
             bytes = -1
             len_update = 4
+        elif base_type == "of_port_desc_t":
+            # This is a special case: it has non-zero min length
+            # but is variable length
+            bytes = -1
+            len_update = of_g.base_length[(base_class, wire_version)]
         elif base_type in of_g.of_base_types:
             bytes = of_g.of_base_types[base_type]["bytes"]
         else: