Issue commented event in a workflow transition does not allow empty comment

I created a worfklow transition with event "Issue Commented" instead of generic event and a transitition view with "Add comment and Assign".

If I use this transition without entering any comment (leave textfield blank), a Velocity exception is thrown (see attachment). The notification mail contains the same stacktrace.

If one use the same transition and enters a comment, eveything work as expected.