SUBSTITUTE formula giving unexpected result

I have an assignment for class that requires me to do certain things in excel and after working on it for around 15 hours or so didn't have any issues until I got to the SUBSTITUTE formula. Basically there is a table with an address column and we are to use the SUBSTITUTE formula to replace "Dr" with "Drive". I figured simple enough, this will be another one of those 30 second parts. 3 or 4 hours later I think I am losing it.

So in cell D18 it currently has "20511 Lake Forest Dr" and when I use the formula =SUBSTITUTE(D18,"Dr","Drive") it puts "0" in the cell instead of what I would expect. If I copy the entire address column to a blank column, say L for example, and use L18 in the formula instead of D18 in cell D18 it works, but I suspect this is not what the professor wants. Any help would be greatly appreciated.