diff --git a/filling.c b/filling.c
index 3231384..a62161a 100644
--- a/filling.c
+++ b/filling.c
@@ -310,7 +310,7 @@ static void print_board(int *board, int w, int h) {
 static game_state *new_game(midend *, const game_params *, const char *);
 static void free_game(game_state *);
 
-#define SENTINEL sz
+#define SENTINEL (sz+1)
 
 static bool mark_region(int *board, int w, int h, int i, int n, int m) {
     int j;