From 9a26f5059be48b19d821c17fe1066c05d167bc7d Mon Sep 17 00:00:00 2001
From: Timothee Cour <timothee.cour2@gmail.com>
Date: Fri, 30 Jul 2021 00:30:22 -0700
Subject: [PATCH] refs #16613: check opcWrDeref for nil (#18613)

---
 compiler/vm.nim | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/compiler/vm.nim b/compiler/vm.nim
index 9541af701a45b..73a228b6c5bcb 100644
--- a/compiler/vm.nim
+++ b/compiler/vm.nim
@@ -812,11 +812,10 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
         # vmgen generates opcWrDeref, which means that we must dereference
         # twice.
         # TODO: This should likely be handled differently in vmgen.
-        if (nfIsRef notin regs[ra].nodeAddr[].flags and
-            nfIsRef notin n.flags):
-          regs[ra].nodeAddr[][] = n[]
-        else:
-          regs[ra].nodeAddr[] = n
+        let nAddr = regs[ra].nodeAddr
+        if nAddr[] == nil: stackTrace(c, tos, pc, "opcWrDeref internal error") # refs bug #16613
+        if (nfIsRef notin nAddr[].flags and nfIsRef notin n.flags): nAddr[][] = n[]
+        else: nAddr[] = n
       of rkRegisterAddr: regs[ra].regAddr[] = regs[rc]
       of rkNode:
          # xxx: also check for nkRefTy as in opcLdDeref?