How to fix Value cannot be null parameter name: connection

I have a controller Web api with c# asp.net, but when I do the request (Post) with transaction, I have this error:

System.Data.Entity.Core.EntityException> (The underlying provider failed on Rollback) value cannot be null, parameter name:connection.

My connection is fine in the file "Web.config", I don't know what the underlying problem is. The database SQL is on a cloud server. I tried a local database and it didn't give me this problem.