Null Reference in Convert file in 2019

This basic project automation line of code works in 2017 but not in 2019 why not?

var convertTask = project.RunAutomaticTask(fileIds, AutomaticTaskTemplateIds.ConvertToTranslatableFormat);

I can confirm that the fileIds are populated from the project and are correct. The project variable is instantiated with a valid project.

I get a null reference exception "Object not set to reference of Object" and in the exception information I can see that the fileId is indeed populated and correct.

When I switch all my .dll references and path back to 2017 Trados and change no code, everything works fine.