Crystal Report Problem

Question

I have used this "Sum(ToNumber({FinalMeter.ExpectedMeter}))" line in my Crystal Report Formula Field to find summation of a particular field and  after saving this line an error comes as "a Field  is required here" in formula workshop form.I can't understand here what do i need to add extra field here as syntax of code suggests that everything is right here.
Please Help me in this Crystal Report Syntax.

Thanks