diff --git a/src/gc.c b/src/gc.c
index af3e56fe..92dc16d9 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -162,7 +162,9 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     for (i = 0; i < env->num_val; ++i) {
       gc_mark(pic, env->values[i]);
     }
-    gc_mark_object(pic, (struct pic_object *)env->up);
+    if (env->up) {
+      gc_mark_object(pic, (struct pic_object *)env->up);
+    }
     break;
   }
   case PIC_TT_PROC: {
@@ -222,7 +224,9 @@ gc_mark_phase(pic_state *pic)
 
   /* callinfo */
   for (ci = pic->ci; ci != pic->cibase; --ci) {
-    gc_mark_object(pic, (struct pic_object *)ci->env);
+    if (ci->env) {
+      gc_mark_object(pic, (struct pic_object *)ci->env);
+    }
   }
 
   /* arena */
diff --git a/src/vm.c b/src/vm.c
index 4c4c2674..d6685f41 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -317,6 +317,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
       ci->argc = pc->u.i;
       ci->pc = pc;
       ci->fp = pic->sp - pc->u.i;
+      ci->env = NULL;
       if (pic_proc_cfunc_p(c)) {
 	v = proc->u.cfunc(pic);
 	pic->sp = ci->fp;