diff --git a/lib/Sema/TypeCheckType.cpp b/lib/Sema/TypeCheckType.cpp
index a0a7b53286d1..900b79ef7d23 100644
--- a/lib/Sema/TypeCheckType.cpp
+++ b/lib/Sema/TypeCheckType.cpp
@@ -5391,10 +5391,10 @@ TypeResolver::resolveOwnershipTypeRepr(OwnershipTypeRepr *repr,
 NeverNullType
 TypeResolver::resolveIsolatedTypeRepr(IsolatedTypeRepr *repr,
                                       TypeResolutionOptions options) {
-  // isolated is only value for non-EnumCaseDecl parameters.
+  // isolated is only valid for non-EnumCaseDecl parameters.
   if ((!options.is(TypeResolverContext::FunctionInput) ||
-       options.hasBase(TypeResolverContext::EnumElementDecl)) &&
-      !options.is(TypeResolverContext::Inherited)) {
+       (options.hasBase(TypeResolverContext::EnumElementDecl) &&
+      !options.is(TypeResolverContext::Inherited)) || options.is(TypeResolverContext::Inherited))) {
     diagnoseInvalid(
         repr, repr->getSpecifierLoc(), diag::attr_only_on_parameters,
         "isolated");
diff --git a/test/Concurrency/isolated_parameters.swift b/test/Concurrency/isolated_parameters.swift
index 531c91e69944..31e99fb79d42 100644
--- a/test/Concurrency/isolated_parameters.swift
+++ b/test/Concurrency/isolated_parameters.swift
@@ -343,3 +343,8 @@ struct WritableActorKeyPath<Root: Actor, Value>: Sendable {
         nonmutating set { setter(root, newValue) }
     }
 }
+
+protocol P {}
+struct S: isolated P {} // expected-error {{'isolated' may only be used on parameters}}
+
+