Power Pivot Measure so I can put a text field in If condition is not true

Dear All

I was wondering whether some one can help me with the problem I have in my power pivot \Measure:

I have the following formula
if('Dept No'[Average Customer KPI]>0,RANKX(All('Dept No'[Department]),'Dept No'[Average Customer KPI]),0)

What I need is if the Average Customer KPI is 0 then rank function to ignore it, so I need to put "N/A" at the end of my formula instead of 0, but when I do this I get an error saying

Calculation error in measure 'Dept No'[261ccda5-8527-47f2-b552-3cb3cb91ebaf]: The second and third arguments of function IF have different data types. This is not supported.

Is there anyway round this so I can put a text if condition is False so Rank function ignores it.

Any help is Appreciate it