diff --git a/lib/libzfs/libzfs_util.c b/lib/libzfs/libzfs_util.c
index 86ed001ad7..8ea08bdf15 100644
--- a/lib/libzfs/libzfs_util.c
+++ b/lib/libzfs/libzfs_util.c
@@ -1203,8 +1203,9 @@ zcmd_write_nvlist_com(libzfs_handle_t *hdl, uint64_t *outnv, uint64_t *outlen,
     nvlist_t *nvl)
 {
 	char *packed;
+	size_t len;
 
-	size_t len = fnvlist_size(nvl);
+	verify(nvlist_size(nvl, &len, NV_ENCODE_XDR) == 0);
 	packed = zfs_alloc(hdl, len);
 
 	verify(nvlist_pack(nvl, &packed, &len, NV_ENCODE_XDR, 0) == 0);