Unable to enter open-ended sickness during maternity

Scenario:

An employee has returned from Maternity and is still in the Maternity Pay Period when they go on sickness absence. As they are not eligible for SSP within this period, the employee must have a Sickness During Maternity absence.

Issue:

We do not know when the employee will be back from sickness absence, so in the first instance when submitting a Sickness During Maternity this would be submitted as 'open ended'. The problem is that we keep getting an error (below) that this isn't possible because the end date must be within the maternity pay period. Surely this error should only trigger when an actual end date is submitted? Can anyone advise on how they have overcome this problem?