stack may be uncleared

src/gc.c

@@ -441,7 +441,16 @@ mark_context_stack(mrb_state *mrb, struct mrb_context *c)
   if (c->ci) e += c->ci->nregs;
   if (c->stbase + e > c->stend) e = c->stend - c->stbase;
   for (i=0; i<e; i++) {
-    mrb_gc_mark_value(mrb, c->stbase[i]);
+    mrb_value v = c->stbase[i];
+
+    if (!mrb_immediate_p(v)) {
+      if (mrb_basic_ptr(v)->tt == MRB_TT_FREE) {
+        c->stbase[i] = mrb_nil_value();
+      }
+      else {
+        mrb_gc_mark(mrb, mrb_basic_ptr(v));
+      }
+    }
   }
 }