WCF Web Service NULL values

Hi

I'm having a WCF Web Service, created from PB .NET 12.5.2 , running on IIS7.

If I receive a message with an empty tag ex. <decimal_value/> or <decimal_value></decimal_value>

The service failes with an error like: "The value '' cannot be parsed as the type 'decimal'

How can I set up the service to accept NULL values?

Thanks!