Infinity issue in SSRS

I have the following expression in an SSRS report:

=Sum(Fields!column1.Value)  * 3600 / ( Sum(Fields!column2.Value) + Sum(Fields!column3.Value) )

when the report runs I get infinity for the column when there is a zero in any of the columns calculated in the expression

Could anyone help me on how to modify the expression to handle the zero values?

Thanks in advance

Chris