C# Array Index Error

Simple array I'm adding to, however I keep getting this Index Out Of Bounds error. Looked around, can't quite figure out where I'm going wrong, I'm assuming it's something to do with where I've used the pos variable. Posted anything I think might be relevant. Thanks in advance.

const int MAX = 5;
    Account[] db = new Account[MAX];

    // global variable for position in array
    int pos;

    private void Form1_Load(object sender, EventArgs e)
    {
        // initialise the array with instantiated objects
        for (int i = 0; i < MAX; i++)
            db[i] = new Account();
    }

private void OpenAcc_Click(object sender, EventArgs e)
    {
        //hide menu
        hide_menu();

        //make new form elements visible
        tbNameEnt.Visible = true;
        tbBalaEnt.Visible = true;
        SubDet.Visible = true;

        //set pos to the first empty element of array
        pos = Array.FindIndex(db, i => i == null);
    }

private void SubDet_Click(object sender, EventArgs e)
    {
        string textBox1;
        double textBox2;
        int a = 0011228;
        int b = pos + 1;
        int accNo;

        //get and parse input details
        textBox1 = tbNameEnt.Text;
        textBox2 = double.Parse(tbBalaEnt.Text);

        //allocate account number
        accNo = int.Parse(a.ToString() + b.ToString());

        //set details for new object in array
        db[pos].SetAccount(textBox1, accNo, textBox2); //ERROR HERE

        //print account created confirmation message
        ConfMess();

        //OK button then takes us back to menu
    }