Cannot convert String null to Numeric

Cannot convert String null to Numeric

Hi,
I've an application in which I have a field that I fill with numeric values. In on user input I set this fiel variable wiht:

    if (input.Costo  ==  null)
    {
        input.Costo = 0;
    }

The application run, But system still return this error to me. If I cancel the Alert, application run correctly:

Error details:

Error occured while executing on user input script.

Unable to update variable Costo

Invalid Numeric Format found

Cannot convert the String null to Numeric

My application: here

Thanks
Mic