not required custom fields with min length

Hello list,

I just installed MantisBT for the first time and my first impression is
really good. I love it from the start.

My question:

I added a custom field (reference number to another ticket system),
which is optional (because there is not neccessarily a ticket in the
other system).

Now I want to define the length of the number as a trivial test. But
that does not work; as it seems "optional" and "minimal length" do not
work together.

How can I configure that? If not possible I would like to make that a
feature request: Accept an empty entry if optional, even if there are
other constraints.

Regards,

Emil

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