Error when allocating users to service groups and institutions

Version: master (15.04)
Platform, brower: any

When allocating an EMPTY username to service groups and institutions in the Web service configuration page, section "Manage service access tokens" and "Manage service users", I got

[WAR] d3 (lib/errors.php:747) Failed to get a recordset: postgres8 error: [-1: ERROR: invalid input syntax for integer: ""] in EXECUTE("SELECT * FROM "usr" WHERE "id" = ? LIMIT 2")
Command was: SELECT * FROM "usr" WHERE "id" = ? and values was ()
Call stack (most recent first):
log_message("Failed to get a recordset: postgres8 error: [-1: E...", 8, true, true) at /home/sonn/code/mahara/master/htdocs/lib/errors.php:97
log_warn("Failed to get a recordset: postgres8 error: [-1: E...") at /home/sonn/code/mahara/master/htdocs/lib/errors.php:747
SQLException->__construct("Failed to get a recordset: postgres8 error: [-1: E...") at /home/sonn/code/mahara/master/htdocs/lib/dml.php:480
get_recordset_sql("SELECT * FROM "usr" WHERE "id" = ? ", array(size 1), 0, 2) at /home/sonn/code/mahara/master/htdocs/lib/dml.php:314
get_record_sql("SELECT * FROM "usr" WHERE "id" = ? ", array(size 1)) at /home/sonn/code/mahara/master/htdocs/lib/dml.php:291
get_record("usr", "id", "") at /home/sonn/code/mahara/master/htdocs/webservice/admin/index.php:212
webservices_user_submit(object(Pieform), array(size 4)) at Unknown:0
call_user_func_array("webservices_user_submit", array(size 2)) at /home/sonn/code/mahara/master/htdocs/lib/pieforms/pieform.php:532
Pieform->__construct(array(size 6)) at /home/sonn/code/mahara/master/htdocs/lib/pieforms/pieform.php:162
Pieform::process(array(size 6)) at /home/sonn/code/mahara/master/htdocs/lib/pieforms/pieform.php:71
pieform(array(size 6)) at /home/sonn/code/mahara/master/htdocs/webservice/admin/index.php:937
service_users_edit_form() at /home/sonn/code/mahara/master/htdocs/webservice/admin/index.php:1074
get_config_options_extended() at /home/sonn/code/mahara/master/htdocs/webservice/admin/index.php:27