CiviMember refusing a valid date

Creating a membership type with w fixed valid period. The from and to dates are refused while they are valid and the error message is displayed.
I think the problem is within Forms\MembershipTypes.php

if( ( $params['period_type'] == 'fixed' ) &&
( $params['duration_unit'] == 'year' ) ) {
$periods = array('fixed_period_start_day', 'fixed_period_rollover_day');
foreach ( $periods as $period ) {
$month = $params[$period]['M'];
$date = $params[$period]['d'];

$month = $params[$period]['M']; should be $month = $params[$period]['m']; (m not capitalized)