Migration for a duplicated Project

Question

I have a web application project with local database (code first).

I duplicated the project and wanting to use the add-migration tool in order to continue development in the duplicated app.

after creating the new migration, on 'update-database'  i'm getting this error:
"Cannot insert the value NULL into column 'ContextKey', table 'mydb.dbo.__MigrationHistory'; column does not allow nulls. INSERT fails.
The statement has been terminated."

How can i continue using migration on my duplicated app?

Thanks