diff --git a/src/Mono.cs b/src/Mono.cs
index 4bdb1fd..313c2f2 100644
--- a/src/Mono.cs
+++ b/src/Mono.cs
@@ -235,6 +235,7 @@ static class Mono
             }
             if (viable_step)
             {
+                tail_fill[min_cell] = -1; // prevent backtracking
                 cell = min_cell;
             }
             // if (min_dist <= 1)