diff --git a/Source/cmCTest.cxx b/Source/cmCTest.cxx
index 73b8e5fcd5..c3d09143d9 100644
--- a/Source/cmCTest.cxx
+++ b/Source/cmCTest.cxx
@@ -2533,8 +2533,8 @@ int cmCTest::Run(std::vector<std::string> const& args)
     CommandArgument{ "-I", CommandArgument::Values::One, dashI },
     CommandArgument{ "--tests-information", CommandArgument::Values::One,
                      dashI },
-    CommandArgument{ "-U", CommandArgument::Values::One, dashU },
-    CommandArgument{ "--union", CommandArgument::Values::One, dashU },
+    CommandArgument{ "-U", CommandArgument::Values::Zero, dashU },
+    CommandArgument{ "--union", CommandArgument::Values::Zero, dashU },
     CommandArgument{ "-R", CommandArgument::Values::One, dashR },
     CommandArgument{ "--tests-regex", CommandArgument::Values::One, dashR },
     CommandArgument{ "-E", CommandArgument::Values::One, dashE },