diff --git a/convert.py b/convert.py
index 77a7222..e4d3091 100644
--- a/convert.py
+++ b/convert.py
@@ -67,6 +67,8 @@ def main():
         screen = screen_py.DHGR560NTSCScreen(palette)
     else:
         screen = screen_py.DHGR560Screen(palette)
+    if args.lookahead < 1:
+        parser.error('--lookahead must be at least 1')
     lookahead = args.lookahead
 
     # Conversion matrix from RGB to CAM16UCS colour values.  Indexed by