Index: sys/netinet6/in6_ifattach.c
===================================================================
--- sys/netinet6/in6_ifattach.c
+++ sys/netinet6/in6_ifattach.c
@@ -481,9 +481,11 @@
 		return (-1);
 	}
 
-	ia = in6ifa_ifpforlinklocal(ifp, 0); /* ia must not be NULL */
-	KASSERT(ia != NULL, ("%s: ia == NULL, ifp=%p", __func__, ifp));
-
+	ia = in6ifa_ifpforlinklocal(ifp, 0);
+	if (ia == NULL) {
+		/* This should be rare, but it happens. */
+		return (-1);
+	}
 	ifa_free(&ia->ia_ifa);
 
 	/*