problem in adding assembly to GAC

Dear friends,
I am tring to add my assembly into GAC folder for that I have given a Strong Name using sn.exe command
> sn.exe -k C:\Inetpub\wwwroot\webcsharp\bin\webcsharp.dll. afer that i am tring to add in GAC using
> Gacutil /i C:\Inetpub\wwwroot\webcsharp\bin\webcsharp.dll.
it is giving methe error like this
"Failure adding assembly to the cache: An attempt was made to load a program with an incorrect format."

can anybody tell me the cause of this problem also it is not allowing the drag and drop of the assembly into this folder.

please help , it will greatly appreciated.

Thanks