diff --git a/py_gen/templates/_pack.py b/py_gen/templates/_pack.py
index b6e8ef7..534e930 100644
--- a/py_gen/templates/_pack.py
+++ b/py_gen/templates/_pack.py
@@ -57,12 +57,11 @@
 :: if length_member_index != None:
         length = sum([len(x) for x in packed])
 :: if ofclass.name == 'of_action_set_field':
-        pad_len = (length + 7)/8*8 - length
-        length += pad_len
-        packed.append('\x00' * pad_len)
+        packed.append(loxi.generic_util.pad_to(8, length))
+        length += len(packed[-1])
 :: #endif
         packed[${length_member_index}] = ${gen_pack_expr(length_member.oftype, 'length')}
 :: #endif
 :: if ofclass.name == 'of_match_v3':
-        packed.append('\x00' * ((length + 7)/8*8 - length))
+        packed.append(loxi.generic_util.pad_to(8, length))
 :: #endif
diff --git a/py_gen/templates/generic_util.py b/py_gen/templates/generic_util.py
index 53091ed..eca828c 100644
--- a/py_gen/templates/generic_util.py
+++ b/py_gen/templates/generic_util.py
@@ -63,6 +63,13 @@
         return deserializer(reader.slice(length), typ)
     return unpack_list(reader, wrapper)
 
+def pad_to(alignment, length):
+    """
+    Return a string of zero bytes that will pad a string of length 'length' to
+    a multiple of 'alignment'.
+    """
+    return "\x00" * ((length + alignment - 1)/alignment*alignment - length)
+
 class OFReader(object):
     """
     Cursor over a read-only buffer