To reproduce,

• Use a course with some enrolled users
• Create an assignment, accept all defaults but in Grade section set "Use marking workflow" to "Yes"
• Display the assignment
• Click the "View/grade all submissions" link
• Click the 'select' box in the heading to select all users
• The in the "with selected" drop down choose "Set marking workflow state" and click Go
• A confirmation pop up appears, select OK

Error....

Debug info: ERROR: invalid input syntax for integer: ""
SELECT * FROM mdl_user WHERE id = $1
[array (
0 => '',
)]
Error code: dmlreadexception
Stack trace:
line 441 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 239 of /lib/dml/pgsql_native_moodle_database.php: call to moodle_database->query_end()
line 744 of /lib/dml/pgsql_native_moodle_database.php: call to pgsql_native_moodle_database->query_end()
line 1415 of /lib/dml/moodle_database.php: call to pgsql_native_moodle_database->get_records_sql()
line 1387 of /lib/dml/moodle_database.php: call to moodle_database->get_record_sql()
line 1366 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
line 3397 of /mod/assign/locallib.php: call to moodle_database->get_record()
line 521 of /mod/assign/locallib.php: call to assign->view_batch_set_workflow_state()
line 53 of /mod/assign/view.php: call to assign->view()

There would be no submissions following the above, but I did try it with an assignment having submissions and the result is the same

(NB: I have no idea if this is 'Assignment' or 'Assignment (2.2)'. It's the standard one in 2.6 anyway.