diff --git a/src/mon.c b/src/mon.c
index 71769658e..8b04754d6 100644
--- a/src/mon.c
+++ b/src/mon.c
@@ -964,7 +964,7 @@ mfndpos(mon, poss, info, flag)
 	poolok = is_flyer(mdat) || is_clinger(mdat) ||
 		 (is_swimmer(mdat) && !wantpool);
 	lavaok = is_flyer(mdat) || is_clinger(mdat) || likes_lava(mdat);
-	thrudoor = (flag & (ALLOW_WALL|BUSTDOOR));
+	thrudoor = ((flag & (ALLOW_WALL|BUSTDOOR)) != 0L);
 	if (flag & ALLOW_DIG) {
 	    struct obj *mw_tmp;