Hi Qing,
I am not able to reproduce this error on my machine. The sample custom check is working just fine. I think this could be because your environment is different then mine. I would like to know the following:
1. What is the operating system on your machine?
2. Is the operating system 32 bit or 64 bit?
3. What is the the version of Visual Studio .NET?
2. Have you modified the check? If yes then can you send me your modified solution.
Thanks,
Dheeraj