diff --git a/Source/cmExportCommand.cxx b/Source/cmExportCommand.cxx
index b4f73435af..736694209d 100644
--- a/Source/cmExportCommand.cxx
+++ b/Source/cmExportCommand.cxx
@@ -212,7 +212,7 @@ static bool HandleExportMode(std::vector<std::string> const& args,
   struct ExportArguments
   {
     ArgumentParser::NonEmpty<std::string> ExportSetName;
-    ArgumentParser::NonEmpty<std::string> Namespace;
+    ArgumentParser::MaybeEmpty<std::string> Namespace;
     ArgumentParser::NonEmpty<std::string> Filename;
     ArgumentParser::NonEmpty<std::string> CxxModulesDirectory;
     cm::optional<cmPackageInfoArguments> PackageInfo;