ms crm 2011 - "This process contains a step that references an invalid custom entity. Remove the invalid references and try again."

Hi Everyone,

I've used Visual Studio and the Developer Toolkit to create a plugin for an on-premise Dynamics CRM 2011 installation.  I right-clicked by project and clicked deploy and it's working great.
 
Now I'm trying to create a custom workflow activity.  When I deploy it, it appears in the workflow designer as a step to add.  But, when I try to add it, I get this error message:
 
"This process contains a step that references an invalid custom entity.  Remove the invalid references and try again."
 
I have removed all of my code from the workflow activity (so it refers to literally nothing) and still get the same error.  I've started over several times using different new project and class names, etc. several times, with no luck.
 
Any ideas?

Thanks, Rajeev