Unable to exempt a wages category from Super Guarantee

Hi,

I've created a new Allowance category and I'm unable to exempt it from calculating Super Guarantee.

I go to Payroll Categories - Superannuation - Superannuation Guarantee - Exempt - tick the Allowance - click ok - and the change doesn't save. 

The Superannuation Gurantee is set up as "Minimum Required Rate" and when I tried to change to "Equals 10.5% of Gross Wages" (to see if it makes any difference) - this change doesn't save either. 

Any ideas what's going on?