Replace 0 with a blank cell in Pivot Table when using Calculated Filed formulas.

I have the following formula for data suppression within a pivot table using a calculated field formula:

=IF(Counter<5,0,MROUND(Counter,5))

This returns a 0 for any figure less than 5, as well as rounding data to multiples of 5, which is half of what I need.

I would like for the pivot table to leave the cells blank rather than a 0. I have tried the following which did not work:

=IF(Counter<5,"",MROUND(Counter,5))

Any ideas?
TIA!