relationship API does not return start_date and end_date

If I try to fetch the start date and end date of a relationship, the API doesn't return those dates. E.g. with drush on the test data:

drush cvapi Relationship.get sequential=1 contact_id_a=111 return="start_date,end_date"

returns

Array
(
[is_error] => 0
[version] => 3
[count] => 5
[values] => Array
(
[0] => Array
(
[id] => 22
[contact_id_a] => 111
)

[1] => Array
(
[id] => 24
[contact_id_a] => 111
)

[2] => Array
(
[id] => 25
[contact_id_a] => 111
)

[3] => Array
(
[id] => 28
[contact_id_a] => 111
)

[4] => Array
(
[id] => 204
[contact_id_a] => 111
)

)

)

(as in: there are no dates )