diff --git a/packages/cultnet-ts/src/contracts.ts b/packages/cultnet-ts/src/contracts.ts
index 972a1023..bc6a7b72 100644
--- a/packages/cultnet-ts/src/contracts.ts
+++ b/packages/cultnet-ts/src/contracts.ts
@@ -1,3 +1,4 @@
+import { isNullOrWhiteSpaceCSharp } from "./cultmesh-authority";
 import type { DocumentPayload } from "@gamecult/cultcache-ts/document";
 import Ajv2020, { type ValidateFunction } from "ajv/dist/2020.js";
 
@@ -211,7 +212,7 @@ export function cultNetTextError(error: string): CultNetErrorMessage {
 export function cultNetWriteNotAuthorizedError(recordKey: string, reason?: string | null): CultNetErrorMessage {
   return {
     schemaVersion: "cultnet.error.v0",
-    error: reason && reason.trim() ? `write_not_authorized: ${reason}` : "write_not_authorized",
+    error: typeof reason === "string" && !isNullOrWhiteSpaceCSharp(reason) ? `write_not_authorized: ${reason}` : "write_not_authorized",
     routingHint: null,
     code: "write_not_authorized",
     details: { field: "recordKey", value: recordKey, asOf: null, current: null },
diff --git a/packages/cultnet-ts/test/cultnet-error.test.ts b/packages/cultnet-ts/test/cultnet-error.test.ts
index 41b1698e..e234f105 100644
--- a/packages/cultnet-ts/test/cultnet-error.test.ts
+++ b/packages/cultnet-ts/test/cultnet-error.test.ts
@@ -93,4 +93,7 @@ test("a write_not_authorized refusal with no reason, or a blank one, says only t
   assert.equal(cultNetWriteNotAuthorizedError("k").error, "write_not_authorized");
   assert.equal(cultNetWriteNotAuthorizedError("k", "  ").error, "write_not_authorized");
   assert.equal(cultNetWriteNotAuthorizedError("k", null).error, "write_not_authorized");
+  // C# string.IsNullOrWhiteSpace counts U+0085 as white space and not U+FEFF; JS trim() disagrees on both.
+  assert.equal(cultNetWriteNotAuthorizedError("k", "\u0085").error, "write_not_authorized");
+  assert.equal(cultNetWriteNotAuthorizedError("k", "\ufeff").error, "write_not_authorized: \ufeff");
 });