diff --git a/lgc.h b/lgc.h
index f571fd2b..0508cd1d 100644
--- a/lgc.h
+++ b/lgc.h
@@ -69,13 +69,16 @@
 
 /*
 ** Layout for bit use in 'marked' field. First three bits are
-** used for object "age" in generational mode.
+** used for object "age" in generational mode. Last bit is used
+** by tests.
 */
 #define WHITE0BIT	3  /* object is white (type 0) */
 #define WHITE1BIT	4  /* object is white (type 1) */
 #define BLACKBIT	5  /* object is black */
 #define FINALIZEDBIT	6  /* object has been marked for finalization */
 
+#define TESTBIT		7
+
 
 
 #define WHITEBITS	bit2mask(WHITE0BIT, WHITE1BIT)
diff --git a/ltests.c b/ltests.c
index c0421781..04e8a00a 100644
--- a/ltests.c
+++ b/ltests.c
@@ -522,7 +522,11 @@ static lu_mem checkgraylist (global_State *g, GCObject *o) {
   int total = 0;  /* count number of elements in the list */
   ((void)g);  /* better to keep it available if we need to print an object */
   while (o) {
-    lua_assert(isgray(o) || getage(o) == G_TOUCHED2);
+    lua_assert(!!isgray(o) ^ (getage(o) == G_TOUCHED2));
+    //lua_assert(isgray(o) || getage(o) == G_TOUCHED2);
+    lua_assert(!testbit(o->marked, TESTBIT));
+    if (keepinvariant(g))
+      l_setbit(o->marked, TESTBIT);  /* mark that object is in a gray list */
     total++;
     switch (o->tt) {
       case LUA_VTABLE: o = gco2t(o)->gclist; break;
@@ -556,9 +560,27 @@ static lu_mem checkgrays (global_State *g) {
 }
 
 
-/* Increment 't' if 'o' should be in a gray list */
-#define incifingray(o,t)  \
-  if (isgray(o) || getage(o) == G_TOUCHED2) (t)++
+/*
+** Check whether 'o' should be in a gray list. If so, increment
+** 'count' and check its TESTBIT. (It must have been previously set by
+** 'checkgraylist'.)
+*/
+static void incifingray (global_State *g, GCObject *o, lu_mem *count) {
+  if (!keepinvariant(g))
+    return;  /* gray lists not being kept in these phases */
+  if (o->tt == LUA_VUPVAL) {
+    /* only open upvalues can be gray */
+    lua_assert(!isgray(o) || upisopen(gco2upv(o)));
+    return;  /* upvalues are never in gray lists */
+  }
+  /* these are the ones that must be in gray lists */
+  if (isgray(o) || getage(o) == G_TOUCHED2) {
+    (*count)++;
+    lua_assert(testbit(o->marked, TESTBIT));
+    resetbit(o->marked, TESTBIT);  /* prepare for next cycle */
+  }
+}
+
 
 static lu_mem checklist (global_State *g, int maybedead, int tof,
   GCObject *newl, GCObject *survival, GCObject *old, GCObject *reallyold) {
@@ -566,22 +588,22 @@ static lu_mem checklist (global_State *g, int maybedead, int tof,
   lu_mem total = 0;  /* number of object that should be in  gray lists */
   for (o = newl; o != survival; o = o->next) {
     checkobject(g, o, maybedead, G_NEW);
-    incifingray(o, total);
+    incifingray(g, o, &total);
     lua_assert(!tof == !tofinalize(o));
   }
   for (o = survival; o != old; o = o->next) {
     checkobject(g, o, 0, G_SURVIVAL);
-    incifingray(o, total);
+    incifingray(g, o, &total);
     lua_assert(!tof == !tofinalize(o));
   }
   for (o = old; o != reallyold; o = o->next) {
     checkobject(g, o, 0, G_OLD1);
-    incifingray(o, total);
+    incifingray(g, o, &total);
     lua_assert(!tof == !tofinalize(o));
   }
   for (o = reallyold; o != NULL; o = o->next) {
     checkobject(g, o, 0, G_OLD);
-    incifingray(o, total);
+    incifingray(g, o, &total);
     lua_assert(!tof == !tofinalize(o));
   }
   return total;
@@ -619,7 +641,7 @@ int lua_checkmemory (lua_State *L) {
   /* check 'tobefnz' list */
   for (o = g->tobefnz; o != NULL; o = o->next) {
     checkobject(g, o, 0, G_NEW);
-    incifingray(o, totalshould);
+    incifingray(g, o, &totalshould);
     lua_assert(tofinalize(o));
     lua_assert(o->tt == LUA_VUSERDATA || o->tt == LUA_VTABLE);
   }
diff --git a/testes/nextvar.lua b/testes/nextvar.lua
index 73af77dd..a16d557b 100644
--- a/testes/nextvar.lua
+++ b/testes/nextvar.lua
@@ -88,6 +88,7 @@ for _, sa in ipairs(sizes) do    -- 'sa' is size of the array part
     arr[1 + sa + sh + 1] = "}"
     local prog = table.concat(arr)
     local f = assert(load(prog))
+    collectgarbage("stop")
     f()    -- call once to ensure stack space
     -- make sure table is not resized after being created
     if sa == 0 or sh == 0 then
@@ -97,6 +98,7 @@ for _, sa in ipairs(sizes) do    -- 'sa' is size of the array part
     end
     local t = f()
     T.alloccount();
+    collectgarbage("restart")
     assert(#t == sa)
     check(t, sa, mp2(sh))
   end