diff --git a/test/test_helper.rb b/test/test_helper.rb
index ba64b267..7a907b06 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -23,6 +23,10 @@ class ActiveSupport::TestCase
   parallelize(workers: :number_of_processors)
 
   parallelize_setup do |worker|
+    # Make sure each worker has it's own base path for conversion testing. Sadly
+    # we can't do this in config/environment/test.rb :(
+    TranscodedItem.class.define_method :codec_conversion_base_path,
+                                       ->(codec_conversion) { File.join(TranscodedItem::BASE_PATH, worker, codec_conversion.id) }
     SimpleCov.command_name "#{SimpleCov.command_name}-#{worker}"
   end