Workflow: can't clear field type version[*]

Hi!

I have following rule:

rule Rule name
|| State.becomes({Won't fix}) {
  tags.clear;
  Sprint.clear;
}

so I want to remove all tags and clear the sprint(don't ask why, it's a requirement from above) when the task is done or won't be fixed.
The tags get cleared as intended, but the Sprint, which is version[*], is not touched.

I had no problems clearing an enum type. I guess there is something specific that has to be done with version type? Changing the field type is not an option.

Thank you in advance,
Roman