Being able to use a boolean property as facet for facetrules

Expected:

I would like to create a facetrule that gives read rights on documents that have a boolean property.
For instance

[test:document] > nt:base

• test:searchable (boolean) = 'true' mandatory autocreated

Actual:

You cannot choose anything else but "String, Name or Reference" as hipposys:type for a facetrule. I tried String with a boolean property, but it does not seem to work.