Documented syntax doesn't work:
string output = null;
StoredProcedure.Create("dbo", "MyStoredProc")
.WithOutputParameter("Name", s => output = s);
This way it works:
string output = null;
StoredProcedure.Create("dbo", "MyStoredProc")
.WithOutputParameter<StoredProcedure, string>("Name", s => output = s);