diff --git a/main.cc b/main.cc
index 8046875..1cec77c 100644
--- a/main.cc
+++ b/main.cc
@@ -137,7 +137,9 @@ struct Grid {
       if (grid_[b + y(pos) * length_] != '.') { break; }
       ++b;
     }
-    return b - x(pos);
+    auto len = b - x(pos);
+    auto ye = std::min(y(pos) + len, length_);
+    return ye - y(pos);
   }
 
   /** Get the next position to check.  n is the size of the square we just