Empty guid returned from BooleanDifference

Sometimes rs.BooleanDifference(keep,cut)
returns an empty Guid and still adds a new Brep to the document.

Is this a bug in Rhinocommon?

Rhino 6 SR13 2019-2-27 (Rhino 6, 6.13.19058.371, Git hash:master @ 17cd8b51accbe17dd04b48028f49d5e77a67f17b)

Hi Goswin,
It’s been logged.
Thanks