Developing a Sample PCL and consuming it

Hi
Yesterday I tried to add a reference of a normal class library in a Silverlight client application using a sample PCL but in trying to do so, it gave me an error. The screenshot of the error is as follows:
The error says:
You can only add project references to other Silverlight projects in the solution.
Does anybody have any idea how to get rid of this error? I'll be really thankful to you for your help.
Waiting for your comments
Regards
