Impossible to use Hg Flow with System Mercurial 3.7.1: error 'dict' object has no attribute 'active'

If the option of "Use System Mercurial" is enabled, and you have version 3.7.1 (or possible later version) then trying to, for instance, start a new Release produces the following error:

{{hg flow -y release start Test
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
flow: error: Hg command failed: hg update develop
flow: error: abort: 'dict' object has no attribute 'active'}}

With previous versions of Mercurial it worked fine, and using the "embedded version" works fine too.