diff --git a/rmw_fastrtps_cpp/src/rmw_serialize.cpp b/rmw_fastrtps_cpp/src/rmw_serialize.cpp
index 76c34e09f..a6a0e2ae8 100644
--- a/rmw_fastrtps_cpp/src/rmw_serialize.cpp
+++ b/rmw_fastrtps_cpp/src/rmw_serialize.cpp
@@ -97,6 +97,6 @@ rmw_get_serialized_message_size(
   size_t * /*size*/)
 {
   RMW_SET_ERROR_MSG("unimplemented");
-  return RMW_RET_ERROR;
+  return RMW_RET_UNSUPPORTED;
 }
 }  // extern "C"
diff --git a/rmw_fastrtps_dynamic_cpp/src/rmw_serialize.cpp b/rmw_fastrtps_dynamic_cpp/src/rmw_serialize.cpp
index dbe8442c3..c6d28b0fa 100644
--- a/rmw_fastrtps_dynamic_cpp/src/rmw_serialize.cpp
+++ b/rmw_fastrtps_dynamic_cpp/src/rmw_serialize.cpp
@@ -100,6 +100,6 @@ rmw_get_serialized_message_size(
   size_t * /*size*/)
 {
   RMW_SET_ERROR_MSG("unimplemented");
-  return RMW_RET_ERROR;
+  return RMW_RET_UNSUPPORTED;
 }
 }  // extern "C"