Drill Down with Custom Message

Hi

I have a drill down chart. The x axis has 2 categories - Category and Sub Category.

3 of the Categories do not have a sub category, and the chart is blank when you right click to drill down.

When you drill down to the Sub Category, the X axis changes to say (Blank) Sub Category, and the chart is blank.

Based on the X axis saying (Blank) Sub Category how can I create a custom message to say 'No data in chart'?

I created a card and dropped in a measure. I have tried this measure, but it isn't quite working:

No data = if(count(table 1[Count of Sub Category]=0),"No data available","")

The syntax half works in that it does display the error message, but it does so on both the Category and Sub Category charts. I only want it to display on the Sub Category charts where they are blank.  

Does anyone have an idea?

Thanks