Task-related types not found. Are you missing a reference to AsyncCtpLibrary.dll?

I added AsyncCTP_Silverlight.dll to one of my solutions and I couldn’t build it, as the IDE kept erroring with the following message:

"Task-related types not found. Are you missing a reference to AsyncCtpLibrary.dll?"

The solution, as it turns out is in this post

…try deleting the suo file for the solution…