diff --git a/src/trap.c b/src/trap.c
index 2ffca0de5..af6d9c9c5 100644
--- a/src/trap.c
+++ b/src/trap.c
@@ -5791,6 +5791,7 @@ openholdingtrap(
     if (!which)
         which = t->tseen ? the_your[t->madeby_u]
                          : strchr(vowels, *trapdescr) ? "an" : "a";
+    assert(which != 0);
     if (*which)
         which = strcat(strcpy(whichbuf, which), " ");