Problem with iif expressionstatement in rdlc

I am using the following expression:
=Fields!job_number.Value
It gives either a number or blank.

I want it to give a number or '0' if it is blank.

Therefore I have used the following expression:
=iif(Fields!job_number.Value="",0,Fields!job_number.Value)

This is displaying en error instead of a number, and 0 if it is blank.

I don't know why it is displaying an error.
The filed is numeric, and the format for display is numeric.