diff --git a/magnets.c b/magnets.c
index d190848..d95d146 100644
--- a/magnets.c
+++ b/magnets.c
@@ -520,7 +520,7 @@ nextchar:
      * (i.e. each end points to the other) */
     for (idx = 0; idx < state->wh; idx++) {
         if (state->common->dominoes[idx] < 0 ||
-            state->common->dominoes[idx] > state->wh ||
+            state->common->dominoes[idx] >= state->wh ||
             state->common->dominoes[state->common->dominoes[idx]] != idx) {
             *prob = "Domino descriptions inconsistent";
             goto done;