diff --git a/src/signal.cpp b/src/signal.cpp
index 2d16e9a730..e8a6690fcb 100644
--- a/src/signal.cpp
+++ b/src/signal.cpp
@@ -294,6 +294,7 @@ static SigFlags ExploreSegment(Owner owner)
 					}
 				}
 
+				assert(IsValidDiagDirection(enterdir));
 				TrackBits tracks = GetTrackBits(tile); // trackbits of tile
 				TrackBits tracks_masked = (TrackBits)(tracks & _enterdir_to_trackbits[enterdir]); // only incidating trackbits