diff --git a/src/lj_api.c b/src/lj_api.c
index 2018cb8f..d40ade30 100644
--- a/src/lj_api.c
+++ b/src/lj_api.c
@@ -975,6 +975,7 @@ LUA_API int lua_setmetatable(lua_State *L, int idx)
     /* Flush cache, since traces specialize to basemt. But not during __gc. */
     if (lj_trace_flushall(L))
       lj_err_caller(L, LJ_ERR_NOGCMM);
+    o = index2adr(L, idx);  /* Stack may have been reallocated. */
     if (tvisbool(o)) {
       /* NOBARRIER: basemt is a GC root. */
       setgcref(basemt_it(g, LJ_TTRUE), obj2gco(mt));