diff --git a/src/hooks/useErrorHandler.ts b/src/hooks/useErrorHandler.ts
index a754a1f4..9db572ff 100644
--- a/src/hooks/useErrorHandler.ts
+++ b/src/hooks/useErrorHandler.ts
@@ -72,7 +72,7 @@ function msg(err: TeaError): string {
   case 'spilt-tea-009':
     if (ctx.filename instanceof Path && !ctx.filename.in(usePrefix())) {
       // this yaml is being worked on by the user
-      msg = `${ctx.filename.prettyLocalString()}: ${ctx.cause.message}`
+      msg = `${ctx.filename.prettyLocalString()}: ${ctx.cause?.message ?? 'unknown cause'}`
     } else {
       const attachment = `${ctx.project}: ${ctx.cause.message}`
       msg = undent`