Have field remain blank if sum is zero

Hello, this is probably something simple that I'm missing, but...

 

I have two number fields in a table that I are added together to make a total. If this total is zero - as in £0 - then I would like the field to remain blank, rather than displaying the zero.

I have tried every way I can think of to do this, but I either lose all the values, or have a list of zeros, which makes my report look messy.

 

These are my fields that are added together:

'PO COST'

'SC COST'

 

Both are located in table 'Frameset'

 

Any help is much appreciated!