Can someone help with a multiplaction inside an ifthen statement

Hope someone can provide guidance. Here is my scenario,

I am trying to do the following A , -B , A-B  (where A and B refer to columns). The account called in B was created with a close balance. The first time the formula for B is (Cell[18, A]*-1). Since I just created the account and there is no balance in it, I get the an ERROR (tryingt to multiply an empty cell times a negative value). So I try the alternative using the ifthen statement. If the cell is empty then assign a value of 0, otherewise do the calculation. The statetment looks like