diff --git a/vm.c b/vm.c
index d945fba..dbbdd70 100644
--- a/vm.c
+++ b/vm.c
@@ -854,6 +854,10 @@ rb_vm_cref(void)
 {
     rb_thread_t *th = GET_THREAD();
     rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(th, th->cfp);
+
+    if (cfp == 0) {
+	rb_raise(rb_eRuntimeError, "Can't call on top of Fiber or Thread");
+    }
     return vm_get_cref(cfp->iseq, cfp->lfp, cfp->dfp);
 }
 
@@ -875,6 +879,9 @@ rb_vm_cbase(void)
     rb_thread_t *th = GET_THREAD();
     rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(th, th->cfp);
 
+    if (cfp == 0) {
+	rb_raise(rb_eRuntimeError, "Can't call on top of Fiber or Thread");
+    }
     return vm_get_cbase(cfp->iseq, cfp->lfp, cfp->dfp);
 }
 
@@ -1968,6 +1975,10 @@ m_core_set_postexe(VALUE self, VALUE iseqval)
 	rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(th, th->cfp);
 	VALUE proc;
 
+	if (cfp == 0) {
+	    rb_bug("m_core_set_postexe: unreachable");
+	}
+
 	GetISeqPtr(iseqval, blockiseq);
 
 	blockptr = RUBY_VM_GET_BLOCK_PTR_IN_CFP(cfp);