diff --git a/Directory.cxx b/Directory.cxx
index c9ad026455cd83203a7ad54a2c971304cc25cc2d..4d89ca44eac861977cd927390e46a02047c154cb 100644
--- a/Directory.cxx
+++ b/Directory.cxx
@@ -48,7 +48,7 @@ Directory::~Directory()
 //----------------------------------------------------------------------------
 unsigned long Directory::GetNumberOfFiles()
 {
-  return this->Internal->Files.size();
+  return static_cast<unsigned long>(this->Internal->Files.size());
 }
 
 //----------------------------------------------------------------------------