diff --git a/src/monmove.c b/src/monmove.c
index c9b5b8d45..253ff3f45 100644
--- a/src/monmove.c
+++ b/src/monmove.c
@@ -1817,6 +1817,7 @@ m_move(register struct monst *mtmp, int after)
          * mfndpos) has no effect for normal attacks, though it lets a
          * confused monster attack you by accident.
          */
+        assert(chi >= 0 && chi < SIZE(info));
         if (info[chi] & ALLOW_U) {
             nix = mtmp->mux;
             niy = mtmp->muy;