diff --git a/udis.py b/udis.py
index a77ad88..d1c4f52 100755
--- a/udis.py
+++ b/udis.py
@@ -140,6 +140,7 @@ while True:
 
         if flags & 2 == und and not args.undocumented:
             # currently only handles one-byte undocumented opcodes
+            length = 1
             format = ""
             mnemonic = "???"