diff --git a/src/vm.c b/src/vm.c
index 715c3db6b..ba320d962 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -1333,10 +1333,12 @@ RETRY_TRY_BLOCK:
       int bidx = (argc < 0) ? a+2 : a+n+1;
       struct RProc *m;
       struct RClass *c;
-      mrb_callinfo *ci;
+      mrb_callinfo *ci = mrb->c->ci;
       mrb_value recv, blk;
       mrb_sym mid = syms[GETARG_B(i)];
 
+      mrb_assert(bidx < ci->nregs);
+
       recv = regs[a];
       if (GET_OPCODE(i) != OP_SENDB) {
         SET_NIL_VALUE(regs[bidx]);
@@ -1523,6 +1525,8 @@ RETRY_TRY_BLOCK:
       mrb_value recv, blk;
       mrb_sym mid = ci->mid;
 
+      mrb_assert(bidx < ci->nregs);
+
       if (mid == 0 || !ci->target_class) {
         mrb_value exc = mrb_exc_new_str_lit(mrb, E_NOMETHOD_ERROR, "super called outside of method");
         mrb_exc_set(mrb, exc);