diff --git a/src/box/lua/schema.lua b/src/box/lua/schema.lua
index ab6243fe9a58352b2a10a8895d88d34e10674a24..794615703a59a09af99a4ff387518785ff21ab32 100644
--- a/src/box/lua/schema.lua
+++ b/src/box/lua/schema.lua
@@ -26,9 +26,9 @@ ffi.cdef[[
     box_iterator_free(box_iterator_t *itr);
     /** \endcond public */
     /** \cond public */
-    size_t
+    ssize_t
     box_index_len(uint32_t space_id, uint32_t index_id);
-    size_t
+    ssize_t
     box_index_bsize(uint32_t space_id, uint32_t index_id);
     int
     box_index_random(uint32_t space_id, uint32_t index_id, uint32_t rnd,