diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index d6bf0fdc4a0d2..5ab923532748b 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -8343,6 +8343,7 @@ RawConformanceIsolationRequest::evaluate(
       if (proto->inheritsFrom(sendableMetatype)) {
         bool isPreconcurrency = moduleImportForPreconcurrency(
             proto, conformance->getDeclContext()) != nullptr;
+        isPreconcurrency |= proto->preconcurrency();
         ctx.Diags.diagnose(
             conformance->getLoc(),
             diag::isolated_conformance_to_sendable_metatype,
diff --git a/test/Concurrency/sendable_objc_attr_in_type_context_swift5.swift b/test/Concurrency/sendable_objc_attr_in_type_context_swift5.swift
index 7e9d6c7c0f6ec..d4b1d55ba7305 100644
--- a/test/Concurrency/sendable_objc_attr_in_type_context_swift5.swift
+++ b/test/Concurrency/sendable_objc_attr_in_type_context_swift5.swift
@@ -211,3 +211,7 @@ extension TestDR {
   @_dynamicReplacement(for: test(completion:))
   func __replaceObjCFunc(_: @escaping () -> Void) {} // Ok
 }
+
+@MainActor
+class InvalidIsolated: NSObject, @MainActor P {}
+// expected-warning@-1 {{cannot form main actor-isolated conformance of 'InvalidIsolated' to SendableMetatype-inheriting protocol 'P'}}