PowerPivot calculated field error

Hello,

I have a powerpivot set up using Customer ID as my primary key. I'm adding Gross Sales and Units sold from each table and they look like they're working. I'm also trying to get add the count of transactions but I keep getting an error that says something about the argument cannot work with values of type String. Any suggestions?

disregard, I must have had something in the formula wrong because I tried it again and it worked.