diff -up lua-5.3.4/src/lapi.c.bug6 lua-5.3.4/src/lapi.c
--- lua-5.3.4/src/lapi.c.bug6	2018-02-13 10:18:59.386451581 -0500
+++ lua-5.3.4/src/lapi.c	2018-02-13 10:19:42.700575448 -0500
@@ -533,6 +533,7 @@ LUA_API void lua_pushcclosure (lua_State
   lua_lock(L);
   if (n == 0) {
     setfvalue(L->top, fn);
+    api_incr_top(L);
   }
   else {
     CClosure *cl;
@@ -546,9 +547,9 @@ LUA_API void lua_pushcclosure (lua_State
       /* does not need barrier because closure is white */
     }
     setclCvalue(L, L->top, cl);
+    api_incr_top(L);
+    luaC_checkGC(L);
   }
-  api_incr_top(L);
-  luaC_checkGC(L);
   lua_unlock(L);
 }