diff --git a/Sources/Testing/Running/Runner.swift b/Sources/Testing/Running/Runner.swift
index a6a76189e..9a04f6d7e 100644
--- a/Sources/Testing/Running/Runner.swift
+++ b/Sources/Testing/Running/Runner.swift
@@ -77,7 +77,7 @@ extension Runner {
   /// type at runtime, it may be better-suited for ``Configuration`` instead.
   private struct _Context: Sendable {
     /// A serializer used to reduce parallelism among test cases.
-    var testCaseSerializer: Serializer?
+    var testCaseSerializer: Serializer<Void>?
   }
 
   /// Apply the custom scope for any test scope providers of the traits
diff --git a/Sources/Testing/Support/Serializer.swift b/Sources/Testing/Support/Serializer.swift
index 94f7d4f5b..fee310bc7 100644
--- a/Sources/Testing/Support/Serializer.swift
+++ b/Sources/Testing/Support/Serializer.swift
@@ -42,8 +42,13 @@ var defaultParallelizationWidth: Int {
 /// items do not start running; they must wait until the suspended work item
 /// either returns or throws an error.
 ///
+/// The generic type parameter `T` is unused. It avoids warnings when multiple
+/// copies of the testing library are loaded into a runner process on platforms
+/// which use the Objective-C runtime, due to non-generic actor types being
+/// implemented as classes there.
+///
 /// This type is not part of the public interface of the testing library.
-final actor Serializer {
+final actor Serializer<T> {
   /// The maximum number of work items that may run concurrently.
   nonisolated let maximumWidth: Int