diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
index 0bdd6edee8c..53abf924cc3 100644
--- a/sys/kern/imgact_elf.c
+++ b/sys/kern/imgact_elf.c
@@ -1800,7 +1800,7 @@ __elfN(note_threadmd)(void *arg, struct sbuf *sb, size_t *sizep)
 		buf = NULL;
 	size = 0;
 	__elfN(dump_thread)(td, buf, &size);
-	KASSERT(*sizep == size, ("invalid size"));
+	KASSERT(sb == NULL || *sizep == size, ("invalid size"));
 	if (size != 0 && sb != NULL)
 		sbuf_bcat(sb, buf, size);
 	free(buf, M_TEMP);