diff --git a/dlib/type_safe_union/type_safe_union_kernel.h b/dlib/type_safe_union/type_safe_union_kernel.h
index 36f4ba522..e1c0c5adb 100644
--- a/dlib/type_safe_union/type_safe_union_kernel.h
+++ b/dlib/type_safe_union/type_safe_union_kernel.h
@@ -575,7 +575,7 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    namespace impl
+    namespace impl_tsu
     {
         struct serialize_helper
         {
@@ -605,7 +605,7 @@ namespace dlib
         {
             // save the type_identity
             serialize(item.type_identity, out);
-            item.apply_to_contents(dlib::impl::serialize_helper(out));
+            item.apply_to_contents(dlib::impl_tsu::serialize_helper(out));
         }
         catch (serialization_error& e)
         {