ASP.NET Can't insert null DateTime to Database

I have this method here:

public List<CustomerQuestionsClass> updateCustomersQuestions(List<CustomerQuestionsClass> items)
{
    connection = new SqlConnection(connectionString);
    command = new SqlCommand(@"IF EXISTS(SELECT * FROM CustomerQuestions WHERE CustomerID = @CustomerID AND QuestionID = @QuestionID)
                               BEGIN
                                    UPDATE CustomerQuestions SET selected = @Selected,  ModifiedBy = @ModifiedBy, DateModified = @DateModified WHERE CustomerID = @CustomerID AND QuestionID = @QuestionID
                               END
                               ELSE
                               BEGIN
                                    INSERT INTO CustomerQuestions (CustomerID, QuestionID, selected, CreatedBy, 
                                        DateCreated, ModifiedBy, DateModified, DateCompleted, DueDate) 
                                    VALUES (@CustomerID, @QuestionID, @Selected, @CreatedBy, @DateCreated, 
                                        @ModifiedBy, @DateModified, @DateCompleted, @DueDate)
                               END");

    command.Parameters.Add("@CustomerID", System.Data.SqlDbType.Int);
    command.Parameters.Add("@QuestionID", System.Data.SqlDbType.Int);
    command.Parameters.Add("@Selected", System.Data.SqlDbType.Bit);
    command.Parameters.Add("@CreatedBy", System.Data.SqlDbType.Text);
    command.Parameters.Add("@DateCreated", System.Data.SqlDbType.DateTime);
    command.Parameters.Add("@ModifiedBy", System.Data.SqlDbType.Text);
    command.Parameters.Add("@DateModified", System.Data.SqlDbType.DateTime);
    command.Parameters.Add("@DateCompleted", System.Data.SqlDbType.DateTime);
    command.Parameters.Add("@DueDate", System.Data.SqlDbType.DateTime);

    command.Connection = connection;
    connection.Open();

    for (int i = 0; i < items.Count; i++)
    {
        command.Parameters["@CustomerID"].Value = items[i].CustomerID;
        command.Parameters["@QuestionID"].Value = items[i].QuestionID;
        command.Parameters["@Selected"].Value = items[i].selected;
        command.Parameters["@CreatedBy"].Value = items[i].Username;
        command.Parameters["@DateCreated"].Value = DateTime.Now;
        command.Parameters["@ModifiedBy"].Value = items[i].Username;
        command.Parameters["@DateModified"].Value = DateTime.Now;
        command.Parameters["@DateCompleted"].Value = items[i].DateCompleted.Equals(DateTime.MinValue) ? (DateTime?)null : items[i].DateCompleted;
        command.Parameters["@DueDate"].Value = items[i].DueDate.Equals(DateTime.MinValue) ? (DateTime?)null : items[i].DueDate;
        command.ExecuteNonQuery();
    }

    connection.Close();

    return items;

}

It's trying to do an insert. But I keep getting this error:

The parameterized query '(@CustomerID int,@QuestionID int,@Selected bit,@CreatedBy text,@' expects the parameter '@DateCompleted', which was not supplied.

When I remove everything having to do with DateCompleted the query runs and data gets inserted, but with the DateCompleted it does not work and I get the error above. The value for DateCompleted is null.