Concatenation problem in formula field

I am attempting to create a formula field that takes the first 3 characters of 1 field's contents and concatenates it with a '-' and then the value of another field that contains an autonumber.

So, the formula that I have is:
Concat(Substring(${Potentials.Project Type},0,3),'-',${Potentials.RFP_Hidden})

So, if Potentials.Project Type is "Online" and Potentials.RFP_Hidden is 2, then this field's value should be "Onl-2"

I don't get any syntax errors when I save it, but then the field is blank when I view the Potential Detail.

If somebody could assist me with this, it would be greatly appreciated.