Internal Server Error from Simple Web Service

[Joe H]

I have a simple web service which accepts two integers as input, then adds
them up and returns the sum as output.

This is written in VB.Net.

I go to the test form that the project generates, and I click the Invoke
button without entering any value for either of the two parameters. IIS
returns an "Internal Server Error".

If the code in my webmethod never gets to execute, how can I ever handle the
error properly and issue the SoapException that would be applicable in this
case?

Thanks,
Joe