C# VSTO dynamically add multiple buttons to a word document

I am working on creating a VSTO addin for MS Word. I am trying to add multiple buttons to a word document with a loop. The code works fine if I just add one button, when I try to add multiple buttons I get the following error:

System.Runtime.InteropServices.COMException
  HResult=0x80004005
  Message=Error HRESULT E_FAIL has been returned from a call to a COM component.
  Source=<Cannot evaluate the exception source>
  StackTrace:Cannot evaluate the exception stack trace

Here is the code:

private void cb_create_click(object sender, RibbonControlEventArgs e)
{

    Document vstoDocument = Globals.Factory.GetVstoObject(Globals.ThisAddIn.Application.ActiveDocument);

    int pos = 0;

    for (int i = 0; i<3; i++)
    {

        Microsoft.Office.Tools.Word.Controls.Button button = new Microsoft.Office.Tools.Word.Controls.Button();

        button = vstoDocument.Controls.AddButton(Globals.ThisAddIn.Application.ActiveDocument.Range(pos, pos), 20, 20, "button_" + pos);
        button.Text = "+";
        button.BackColor = System.Drawing.Color.Azure;
        button.Click += Button_Click;
        pos++;
    }

}

 private void Button_Click(object sender, EventArgs e)
{
    //other code goes here.
}