diff --git a/module/zfs/ddt_zap.c b/module/zfs/ddt_zap.c
index 280bff35a0b8..6fb6ea2a3a92 100644
--- a/module/zfs/ddt_zap.c
+++ b/module/zfs/ddt_zap.c
@@ -70,7 +70,7 @@ ddt_zap_lookup(objset_t *os, uint64_t object, ddt_entry_t *dde)
 		goto out;
 
 	ASSERT(one == 1);
-	ASSERT(csize <= sizeof (cbuf));
+	ASSERT(csize <= (sizeof (dde->dde_phys) + 1));
 
 	error = zap_lookup_uint64(os, object, (uint64_t *)&dde->dde_key,
 	    DDT_KEY_WORDS, 1, csize, cbuf);