diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index 05977fb759a..b10f9505473 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -7942,7 +7942,7 @@ ConformanceIsolationRequest::evaluate(Evaluator &evaluator, ProtocolConformance
     return ActorIsolation::forNonisolated(false);
 
   // If we are inferring isolated conformances and the conforming type is
-  // isolated to a global actor,
+  // isolated to a global actor, use the conforming type's isolation.
   auto nominal = dc->getSelfNominalTypeDecl();
   if (ctx.LangOpts.hasFeature(Feature::InferIsolatedConformances) &&
       nominal) {
diff --git a/lib/Serialization/Deserialization.cpp b/lib/Serialization/Deserialization.cpp
index 9ef86d0c701..911a654d323 100644
--- a/lib/Serialization/Deserialization.cpp
+++ b/lib/Serialization/Deserialization.cpp
@@ -1058,8 +1058,11 @@ ProtocolConformanceDeserializer::readNormalProtocolConformance(
   auto globalActorType = globalActorTypeOrError.get();
 
   TypeExpr *globalActorTypeExpr = nullptr;
-  if (globalActorType)
+  if (globalActorType) {
     globalActorTypeExpr = TypeExpr::createImplicit(globalActorType, ctx);
+    rawOptions |=
+        static_cast<unsigned>(ProtocolConformanceFlags::GlobalActorIsolated);
+  }
 
   auto conformance = ctx.getNormalConformance(
       conformingType, proto, SourceLoc(), dc,
diff --git a/test/Serialization/Inputs/def_isolated_conformance.swift b/test/Serialization/Inputs/def_isolated_conformance.swift
index a339a12a777..f908f1eac16 100644
--- a/test/Serialization/Inputs/def_isolated_conformance.swift
+++ b/test/Serialization/Inputs/def_isolated_conformance.swift
@@ -8,3 +8,8 @@ public class MyClass { }
 extension MyClass: @MainActor MyProtocol {
   @MainActor public func f() { }
 }
+
+public protocol OtherProtocol {
+}
+
+extension MyClass: OtherProtocol { }
diff --git a/test/Serialization/isolated_conformance.swift b/test/Serialization/isolated_conformance.swift
index 064bdf03345..d7e7701313c 100644
--- a/test/Serialization/isolated_conformance.swift
+++ b/test/Serialization/isolated_conformance.swift
@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module -target %target-swift-5.1-abi-triple -swift-version 6 -o %t/def_isolated_conformance.swiftmodule %S/Inputs/def_isolated_conformance.swift
+// RUN: %target-swift-frontend -emit-module -target %target-swift-5.1-abi-triple -swift-version 6 -o %t/def_isolated_conformance.swiftmodule %S/Inputs/def_isolated_conformance.swift -default-isolation=MainActor
 
 // RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 %s -I %t
 
@@ -8,8 +8,11 @@
 import def_isolated_conformance
 
 func acceptMyProtocol(_: some MyProtocol) { }
+func acceptOtherProtocol(_: some MyProtocol) { }
 
 nonisolated func f(mc: MyClass) {
   acceptMyProtocol(mc)
   // expected-error@-1{{main actor-isolated conformance of 'MyClass' to 'MyProtocol' cannot be used in nonisolated context}}
+  acceptOtherProtocol(mc)
+  // expected-error@-1{{main actor-isolated conformance of 'MyClass' to 'MyProtocol' cannot be used in nonisolated context}}
 }