diff --git a/src/lj_ccallback.c b/src/lj_ccallback.c
index a7979a5d..15346775 100644
--- a/src/lj_ccallback.c
+++ b/src/lj_ccallback.c
@@ -722,6 +722,7 @@ lua_State * LJ_FASTCALL lj_ccallback_enter(CTState *cts, void *cf)
   lua_State *L = cts->L;
   global_State *g = cts->g;
   lj_assertG(L != NULL, "uninitialized cts->L in callback");
+  lj_assertG(!isdead(g, obj2gco(L)), "dead cts->L in callback");
   if (tvref(g->jit_base)) {
     setstrV(L, L->top++, lj_err_str(L, LJ_ERR_FFI_BADCBACK));
     if (g->panic) g->panic(L);
diff --git a/src/lj_gc.c b/src/lj_gc.c
index 3ca6cf1b..244340c1 100644
--- a/src/lj_gc.c
+++ b/src/lj_gc.c
@@ -629,6 +629,10 @@ static void atomic(global_State *g, lua_State *L)
   setgcrefnull(g->gc.weak);
   lj_assertG(!iswhite(obj2gco(mainthread(g))), "main thread turned white");
   gc_markobj(g, L);  /* Mark running thread. */
+#if LJ_HASFFI
+  if (ctype_ctsG(g) && ctype_ctsG(g)->L)  /* Mark cts->L thread. */
+    gc_markobj(g, ctype_ctsG(g)->L);
+#endif
   gc_traverse_curtrace(g);  /* Traverse current trace. */
   gc_mark_gcroot(g);  /* Mark GC roots (again). */
   gc_propagate_gray(g);  /* Propagate all of the above. */