Brigantia install fails with message when attempting to reformat existing partition as btrfs

I booted from a Trisquel 5.5 amd64 20120311 iso DVD.
Selected English then Try Trisquel without Installing (without Accessibility)
When the desktop appeared I double clicked Install Trisquel 5.5
Selected English language
Clicked continue / next at the install requirements screen which all had green check marks.
Selected Something Else as my disk partitioning / install choice
On an existing drive I chose 2 Linux existing ext4 partitions to be / and /home respectively, both to be formatted as btrfs
As I was waiting for the installer to go beyond the location dialog the installer presented the following message in a dialog

"The file system creation in partition #5 of SCSI3 (0,0,0) (sdb) failed."

Clicking on the dialog's one button, or its Close 'X' button would not remove the dialog.

ISTR remember reporting a similar bug against a long past Ubuntu beta when they first included ext4 support and almost identical behavior was caused by part of the relevant fs support not being on the beta CD.