diff --git a/mines.c b/mines.c
index c666001..04364e5 100644
--- a/mines.c
+++ b/mines.c
@@ -435,7 +435,9 @@ static void ss_add(struct setstore *ss, int x, int y, int mask, int mines)
      * Create a set structure and add it to the tree.
      */
     s = snew(struct set);
+    assert(SHRT_MIN <= x && x <= SHRT_MAX);
     s->x = x;
+    assert(SHRT_MIN <= y && y <= SHRT_MAX);
     s->y = y;
     s->mask = mask;
     s->mines = mines;
@@ -506,7 +508,9 @@ static struct set **ss_overlap(struct setstore *ss, int x, int y, int mask)
 	    /*
 	     * Find the first set with these top left coordinates.
 	     */
+            assert(SHRT_MIN <= xx && xx <= SHRT_MAX);
 	    stmp.x = xx;
+            assert(SHRT_MIN <= yy && yy <= SHRT_MAX);
 	    stmp.y = yy;
 	    stmp.mask = 0;