diff --git a/google/cloud/storage/internal/async/object_descriptor_reader_tracing.cc b/google/cloud/storage/internal/async/object_descriptor_reader_tracing.cc
index 725ce5347dff6..39f70fd93011d 100644
--- a/google/cloud/storage/internal/async/object_descriptor_reader_tracing.cc
+++ b/google/cloud/storage/internal/async/object_descriptor_reader_tracing.cc
@@ -57,7 +57,7 @@ class ObjectDescriptorReaderTracing : public ObjectDescriptorReader {
                 "gl-cpp.read-range",
                 {{/*sc::kRpcMessageType=*/"rpc.message.type", "RECEIVED"},
                  {sc::kThreadId, internal::CurrentThreadId()},
-                 {"message.size", payload.size()}});
+                 {"message.size", static_cast<std::uint32_t>(payload.size())}});
           } else {
             span->AddEvent(
                 "gl-cpp.read-range",
diff --git a/google/cloud/storage/internal/async/object_descriptor_reader_tracing_test.cc b/google/cloud/storage/internal/async/object_descriptor_reader_tracing_test.cc
index fb7c71a03dd05..b02e7e1783536 100644
--- a/google/cloud/storage/internal/async/object_descriptor_reader_tracing_test.cc
+++ b/google/cloud/storage/internal/async/object_descriptor_reader_tracing_test.cc
@@ -60,15 +60,16 @@ TEST(ObjectDescriptorReaderTracing, Read) {
 
   auto actual = reader->Read().get();
   auto spans = span_catcher->GetSpans();
-  EXPECT_THAT(spans, ElementsAre(AllOf(
-                         SpanNamed("storage::AsyncConnection::ReadObjectRange"),
-                         SpanHasEvents(AllOf(
-                             EventNamed("gl-cpp.read-range"),
-                             SpanEventAttributesAre(
-                                 OTelAttribute<std::size_t>("message.size", 10),
-                                 OTelAttribute<std::string>(sc::kThreadId, _),
-                                 OTelAttribute<std::string>("rpc.message.type",
-                                                            "RECEIVED")))))));
+  EXPECT_THAT(
+      spans, ElementsAre(
+                 AllOf(SpanNamed("storage::AsyncConnection::ReadObjectRange"),
+                       SpanHasEvents(AllOf(
+                           EventNamed("gl-cpp.read-range"),
+                           SpanEventAttributesAre(
+                               OTelAttribute<std::uint32_t>("message.size", 10),
+                               OTelAttribute<std::string>(sc::kThreadId, _),
+                               OTelAttribute<std::string>("rpc.message.type",
+                                                          "RECEIVED")))))));
 }
 
 TEST(ObjectDescriptorReaderTracing, ReadError) {