diff --git a/lapi.c b/lapi.c
index da866a6..147ed0f 100644
--- a/lapi.c
+++ b/lapi.c
@@ -1213,8 +1213,8 @@
   lua_lock(L);
   o = index2stack(L, idx);
   nresults = L->ci->nresults;
-  api_check(L, L->openupval == NULL || uplevel(L->openupval) < o,
-               "there is an already marked index below");
+  api_check(L, L->openupval == NULL || uplevel(L->openupval) <= o,
+               "marked index below or equal new one");
   luaF_newtbcupval(L, o);  /* create new to-be-closed upvalue */
   if (!hastocloseCfunc(nresults))  /* function not marked yet? */
     L->ci->nresults = codeNresults(nresults);  /* mark it */
diff --git a/lauxlib.c b/lauxlib.c
index 6069ed1..fd4acbd 100644
--- a/lauxlib.c
+++ b/lauxlib.c
@@ -524,8 +524,8 @@
 
 /*
 ** Returns a pointer to a free area with at least 'sz' bytes in buffer
-** 'B'. 'boxidx' is the position in the stack where the buffer's box is
-** or should be.
+** 'B'. 'boxidx' is the relative position in the stack where the
+** buffer's box is or should be.
 */
 static char *prepbuffsize (luaL_Buffer *B, size_t sz, int boxidx) {
   if (B->size - B->n >= sz)  /* enough space? */
@@ -538,8 +538,10 @@
     if (buffonstack(B))  /* buffer already has a box? */
       newbuff = (char *)resizebox(L, boxidx, newsize);  /* resize it */
     else {  /* no box yet */
+      lua_pushnil(L);  /* reserve slot for final result */
       newbox(L);  /* create a new box */
-      lua_insert(L, boxidx);  /* move box to its intended position */
+      /* move box (and slot) to its intended position */
+      lua_rotate(L, boxidx - 1, 2);
       lua_toclose(L, boxidx);
       newbuff = (char *)resizebox(L, boxidx, newsize);
       memcpy(newbuff, B->b, B->n * sizeof(char));  /* copy original content */
@@ -576,8 +578,8 @@
   lua_State *L = B->L;
   lua_pushlstring(L, B->b, B->n);
   if (buffonstack(B)) {
-    resizebox(L, -2, 0);  /* delete old buffer */
-    lua_remove(L, -2);  /* remove its header from the stack */
+    lua_copy(L, -1, -3);  /* move string to reserved slot */
+    lua_pop(L, 2);  /* pop string and box (closing the box) */
   }
 }
 
diff --git a/testes/locals.lua b/testes/locals.lua
index 8766b3d..90a8b84 100644
--- a/testes/locals.lua
+++ b/testes/locals.lua
@@ -242,6 +242,7 @@
   assert(a == 1 and b == 2 and c == 3 and X == 20 and Y == 10 and d == nil)
 end
 
+
 do   -- errors in __close
   local log = {}
   local function foo (err)
@@ -264,7 +265,9 @@
          and #log == 4)
 end
 
+
 if rawget(_G, "T") then
+
   -- memory error inside closing function
   local function foo ()
     local *toclose y = function () T.alloccount() end
@@ -296,7 +299,7 @@
   local function test ()
     local *toclose x = enter(0)   -- set a memory limit
     -- creation of previous upvalue will raise a memory error
-    os.exit(false)    -- should not run
+    assert(false)    -- should not run
   end
 
   local _, msg = pcall(test)
@@ -326,33 +329,54 @@
 
 
   do    -- testing 'toclose' in C string buffer
-    local s = string.rep("a", 10000)
+    collectgarbage()
+    local s = string.rep('a', 10000)    -- large string
+    local m = T.totalmem()
+    collectgarbage("stop")
+    s = string.upper(s)    -- allocate buffer + new string (10K each)
+    -- ensure buffer was deallocated
+    assert(T.totalmem() - m <= 11000)
+    collectgarbage("restart")
+  end
+
+  do   -- now some tests for freeing buffer in case of errors
+    local lim = 10000           -- some size larger than the static buffer
+    local extra = 2000          -- some extra memory (for callinfo, etc.)
+
+    local s = string.rep("a", lim)
+
+    -- concat this table needs two buffer resizes (one for each 's') 
     local a = {s, s}
 
-    -- ensure proper initialization (stack space, metatable)
-    table.concat(a)
-    collectgarbage(); collectgarbage()
+    collectgarbage()
 
-    local m = T.totalmem()
+    m = T.totalmem()
+    collectgarbage("stop")
+
+    -- error in the first buffer allocation
+    T. totalmem(m + extra)
+    assert(not pcall(table.concat, a))
+    -- first buffer was not even allocated
+    assert(T.totalmem() - m <= extra)
 
     -- error in the second buffer allocation
-    T.alloccount(3)
+    T. totalmem(m + lim + extra)
     assert(not pcall(table.concat, a))
-    T.alloccount()
     -- first buffer was released by 'toclose'
-    assert(T.totalmem() - m <= 5000)
+    assert(T.totalmem() - m <= extra)
 
     -- error in creation of final string
-    T.alloccount(4)
+    T.totalmem(m + 2 * lim + extra)
     assert(not pcall(table.concat, a))
-    T.alloccount()
     -- second buffer was released by 'toclose'
-    assert(T.totalmem() - m <= 5000)
+    assert(T.totalmem() - m <= extra)
 
-    -- userdata, upvalue, buffer, buffer, string
-    T.alloccount(5)
-    assert(#table.concat(a) == 20000)
-    T.alloccount()
+    -- userdata, upvalue, buffer, buffer, final string
+    T.totalmem(m + 4*lim + extra)
+    assert(#table.concat(a) == 2*lim)
+
+    T.totalmem(0)     -- remove memory limit
+    collectgarbage("restart")
 
     print'+'
   end