can not compile 64bit a class library project when a web service is refrenced

I have a class library project in VS2010. When I want to compile as 64-bit, the following error raises:

An attempt was made to load an assembly with an incorrect format: D:\ChilkatDotNet4.dll.

But wh开发者_如何学编程en I remove web service from project, it compiles successfully. (The chilkat DLL is 64bit)

Any idea?