diff --git a/include/display.h b/include/display.h
index ee4958905..b97b1d8e4 100644
--- a/include/display.h
+++ b/include/display.h
@@ -104,7 +104,7 @@
  * canseemon() or canspotmon() which already check that.
  */
 #define _see_with_infrared(mon) \
-    (!Blind && Infravision && mon && infravisible(mon->data) \
+    (!Blind && Infravision && infravisible(mon->data) \
      && couldsee(mon->mx, mon->my))
 
 /*
diff --git a/src/vision.c b/src/vision.c
index 341ef9c57..d670c3a38 100644
--- a/src/vision.c
+++ b/src/vision.c
@@ -2131,6 +2131,7 @@ howmonseen(struct monst *mon)
     int xraydist = (u.xray_range < 0) ? -1 : (u.xray_range * u.xray_range);
     unsigned how_seen = 0; /* result */
 
+    /* assert(mon != NULL) */
     /* normal vision;
        cansee is true for both normal and astral vision,
        but couldsee it not true for astral vision */