diff --git a/packages/typespec-client-generator-core/src/lib.ts b/packages/typespec-client-generator-core/src/lib.ts
index 178e82742f..0d49d586d3 100644
--- a/packages/typespec-client-generator-core/src/lib.ts
+++ b/packages/typespec-client-generator-core/src/lib.ts
@@ -169,7 +169,7 @@ export const $lib = createTypeSpecLibrary({
     "flatten-polymorphism": {
       severity: "error",
       messages: {
-        default: `Flattening polymorphic types is not supported.`,
+        default: `Cannot flatten property of polymorphic type.`,
       },
     },
   },