Part defaults to 0 if it is the omitted optional_value

[pedroilidio]

Config:

[bumpversion]
current_version = 0.3.1
commit = True
tag = True
allow_dirty = True
parse = 
    (?P<major>\d+)\.
    (?P<minor>\d+)\.
    (?P<patch>\d+)
    ((?P<release>\D+)(?P<build>\d*))?
serialize = 
    {major}.{minor}.{patch}{release}{build}
    {major}.{minor}.{patch}{release}
    {major}.{minor}.{patch}

[bumpversion:part:release]
optional_value = g
first_value = g
values =
    dev
    a
    b
    g

[bumpversion:part:build]

When running

bumpversion -n --verbose --current-version "0.2.3" build --no-configured-files

I get new_version=0.2.301. Shouldn't we be getting new_version=0.2.3g1 instead?
Nevertheless, I can get desired result changing serialize as below.

serialize = 
    {major}.{minor}.{patch}{release}{build}
    {major}.{minor}.{patch}g{build}
    {major}.{minor}.{patch}{release}
    {major}.{minor}.{patch}