How to mock gorm insert with go-sql (postgres)

I'm using Gorm with the postgresql driver. I try to mock a database insert with go-sqlmock:

type Test struct {
    FirstName string `json:"first_name"`
}

func (db *DB) CreateTest() (*Test, error) {
    test := Test{"c"}
    if result := db.Create(&test); result.Error != nil {
        return nil, result.Error
    }

    return &test, nil
}


func TestCreateUser(t *testing.T) {
    _, mock, _ := sqlmock.NewWithDSN("sqlmock_db_0")

    mockedGorm, _ := gorm.Open("sqlmock", "sqlmock_db_0")
    defer mockedGorm.Close()
    myDB := &DB{mockedGorm}

    mock.ExpectExec("INSERT INTO test").WithArgs("c").WillReturnResult(sqlmock.NewResult(1, 1))
    myDB.Exec("INSERT INTO test(first_name) VALUES (?)", "c")


    if _, err := myDB.CreateTest(); err != nil {
        t.Errorf("error was not expected: %s", err)
    }

    if err := mock.ExpectationsWereMet(); err != nil {
        t.Errorf("there were unfulfilled expectations: %s", err)
    }
}

Unfortunately, this gives me an error:

error was not expected: all expectations were already fulfilled, call to database transaction Begin was not expected

How do I test an insert with gorm, postgresql and sql-mock properly?