diff --git a/DetectorDescription/DDCMS/interface/DDPlugins.h b/DetectorDescription/DDCMS/interface/DDPlugins.h
index 6eb1c0fe47392..7d365f38225cc 100644
--- a/DetectorDescription/DDCMS/interface/DDPlugins.h
+++ b/DetectorDescription/DDCMS/interface/DDPlugins.h
@@ -19,6 +19,7 @@ namespace dd4hep {
 }
 
 namespace {
+  namespace ns { class xml_h; }
   template <typename P, typename S> class Factory;
   DD4HEP_PLUGIN_FACTORY_ARGS_4( long, dd4hep::Detector*,
 				cms::DDParsingContext*, ns::xml_h*,