diff --git a/tools/similarity.c b/tools/similarity.c
index 862b68df..fcf29a83 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -96,7 +96,7 @@ int main(int argc,char **argv)
         if (f1[index] != f2[index]) {
             printf("BASIC does not match MS BASIC at %X (%X != %X)\n", addr, f1[index], f2[index]);
         }
-        f1[index] = 0;
+        f1[index] = 0xFF;
       }
     }
   }