diff --git a/lib/ci/reporter/rake/test_unit.rb b/lib/ci/reporter/rake/test_unit.rb
index d7ece1e..4a478eb 100644
--- a/lib/ci/reporter/rake/test_unit.rb
+++ b/lib/ci/reporter/rake/test_unit.rb
@@ -5,7 +5,11 @@
     task :testunit do
       rm_rf ENV["CI_REPORTS"] || "test/reports"
       test_loader = CI::Reporter.maybe_quote_filename "#{File.dirname(__FILE__)}/test_unit_loader.rb"
-      ENV["TESTOPTS"] = "#{ENV["TESTOPTS"]} #{test_loader}"
+      # Use RUBYOPT instead of TESTOPTS to avoid conflicts with rake 13.4+
+      # which now preserves TESTOPTS and appends -v when verbose is enabled.
+      # Adding a file path to TESTOPTS caused rake_test_loader to treat the
+      # loader file as a test file, resulting in "version unknown" errors.
+      ENV["RUBYOPT"] = "#{ENV["RUBYOPT"]} -r#{test_loader}"
     end
   end
 end