When editing the cell, you can use F4 to toggle through all combinations (AB4, $AB4, AB$4, $AB$4). I don't know if using this tip will solve the problem. Your formula appears correct. I've never seen this type of error. You might try writing the formula in the cell by entering the equal sign with the keyboard, selecting AB4 with the mouse, and using F4 (in this case, 3 times) to set the absolute/relative state you want.

Are you referencing a cell on a separate tab (Master Index)?