WCF Test client problem

hi,

I'm a newbee to WCF and working on a project where i use sql server 2005 >
Linq > WCF
As reference i used the new profession C# 2008 book.
The example that im using works fine.
My project on the other hand did not work.

After examening all the code and running winmerge 10 times or so, some day
later.
I got the code working!!

Problem everytime I ran my project I got an error in the wcf test client.

Wy, well in my database i got a table with a datetime value and it did
allows nulls but the wcf test client can't handel this?

He also does this on money values and im not sure but foreign keys to.
Can someone help me understand this problem?

thanks