Error inserting null value into nullable field using TdCommand

I have the following unit test which is failing due to a TdException with the message "[Teradata Database] [2689] Field is null for column defined as NOT NULL."

[TestMethod]
        public void CanInsertNullStringParameter()
        {
            using (DbConnection connection = db.CreateConnection())
            {
                connection.Open();
                using (DbTransaction transaction = connection.BeginTransaction())
                {
                    string sqlString = "insert into TempCustomers (CustomerID, CompanyName, ContactName) Values (?, ?, ?);"
                    DbCommand insert = db.GetSqlStringCommand(sqlString);
                    db.AddInParameter(insert, "CustomerID", DbType.Int32, 1);
                    db.AddInParameter(insert, "CompanyName", DbType.String, "test");
                    db.AddInParameter(insert, "ContactName", DbType.String, null);

                    db.ExecuteNonQuery(insert, transaction);
                    transaction.Rollback();
                }
            }
        }

The SQL used to create the table is as follows ... I have confirmed via Teradata SQL Assistant that column ContactName is indeed nullable, and that I can insert a null value via an insert statement applied directly in SQL Assistant.

CREATE TABLE TempCustomers (CustomerId VARCHAR(5) NOT NULL, CompanyName VARCHAR(128) NOT NULL, ContactName VARCHAR(128) NULL);

Internally, my C# code gets down to a TdCommand.ExecuteNonQuery(), which is where the exception gets thrown.  The command text is the SQL below. I have confirmed that the third parameter has value DBNull.Value.

insert into TempCustomers (CustomerID, CompanyName, ContactName) Values (?, ?, ?)

Can someone advise why I'm receiving this error?