add custom control to outlook toolbar in VSTO
 

I want to add a custom control to a tool bar in outlook. I am wrting the following code

Office.CommandBarControl ilc_Menu_Send_Progress = (Office.CommandBarControl)ilc_MainMenu.Controls
.Add(Office.MsoControlType.msoControlCustom,(object) testControl1, missing, 1, true);

But i am getting the following error

Type mismatch. (Exception from HRESULT: 0x80020005 (DISP_E_TYPEMISMATCH))