diff --git a/src/gc.c b/src/gc.c
index 62161b7f0..3604066dd 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -658,7 +658,7 @@ gc_mark_children(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
       struct REnv *e = (struct REnv*)obj;
       mrb_int i, len;
 
-      if (MRB_ENV_STACK_SHARED_P(e) && e->cxt->fib) {
+      if (MRB_ENV_STACK_SHARED_P(e) && e->cxt && e->cxt->fib) {
         mrb_gc_mark(mrb, (struct RBasic*)e->cxt->fib);
       }
       len = MRB_ENV_STACK_LEN(e);