Add membership does not work for future start date

[mimorita]

At /staff/members/detail/USER/#membership

For member S.R., tried to add a new membership. S.R. has a current membership with bill date of the 31st, and there is no 31st of November.

"Edit Subscriptions" > "New Package"

Effective Date "Start of Next Bill Period 2018-11-31"
Select Package "PT5"
No end date, no paid by, no other resources

Pressing "Update" brings up the "Edit Subscriptions" popup again, and the new membership never records.

It would be nice if there was an alert that the attempted date does not exist (or if the bill date would not automatically try to add to a nonexistent date).