diff --git a/prjxray/tile_segbits.py b/prjxray/tile_segbits.py
index 3715a1ad..b9f940b7 100644
--- a/prjxray/tile_segbits.py
+++ b/prjxray/tile_segbits.py
@@ -69,7 +69,7 @@ def read_segbits(f):
 
         parts = l.split(' ')
 
-        assert len(parts) > 1
+        assert len(parts) > 1, l
 
         segbits[parts[0]] = [parsebit(val) for val in parts[1:]]