Rounded MT3 - Mesh is not closed error

[phschoen]

I just wanted to generate a mt3 keycap for SLA printing. The preview is working however if I want to generate the STL
the error "the given Mesh is not closed. Unable to convert CGAL_Nerf_Polyhedron"

The key I generated with the following settings:

flat_support() no_stem_support() box_cherry() mt3_row(2) rounded() key();