Hi, we are experiencing some strange behavior with one of our Custom Workflow Activities and looking for some guidance. What we find is that if the Custom Activity is places on Workflow that contains any of the following standard Workflow Activities; Find User, Find Group; Search Repository, then when the Workflow runs, the following error is reported:...

PublicKeyToken=null' is not marked as serializable

On workflows where the above mentioned activities are not present, and only our custom activity, then no errors are reported and the workflow runs normally performing each operation as expected. 

It is almost like there is some code in our custom activity that is not compatible with some of the standard workflow activities and at Run time, throws an exception. 

Any ideas what could cause this or where we can start looking in the code of our Custom Activity?