Number formula problem

I have a spreadsheet that in it I am trying to add a row of numbers. However, some of the cells in the row contain errors because they are 'future' looking column and have formulas in them that are not yet populated. Anyway, I want the sum formula to ignore those cells or at least treat the 'error' as a zero, then sum up the row. Have tried this formula, but it just makes the total sum to be '0'.

Help!

IFERROR(SUM(C52:AR52),0)