How to Remove un-grouped items, or (Blank) items from Key Influencer visual?

I'm not sure if this is possible or not, but is there a way to remove the (Blank) category from the Key Influencer visual?  This category gets created when there are blank entries in the "Explain By" field.  See below for a screen shot

In the abov example, I've grouped asnwer into two categories: Top 2 Box (for values of 4 or 5) and Not Top 2 Box (for values of 0, 1, 2, or 3).  I have some blanks in this field, but I don't want them to be included in the anylsis.  I'm going to be using multiple fields in the "Explain By" section of the visual, so I can't simply filter them out as blanks are not consistent across fields.