Can't set CommandBarButton Tag property?

[280Z28]

I'm getting an exception on the marked line (comment) in the following code
in my add-in, which seems to be correctly written. I know I'm missing
something, probably something simple. I'm trying to add a button control as
the next to last item on the menu. As you can see, I want the Click handler
to know which folder was right-clicked on to execute the command.

***** begin code *****

public void OnConnection( object application, Extensibility.ext_ConnectMode
connectMode, object addInInst, ref System.Array custom )
{
applicationObject = (Outlook.Application)application;
addInInstance = addInInst;

applicationObject.FolderContextMenuDisplay += new
Microsoft.Office.Interop.Outlook.ApplicationEvents_11_FolderContextMenuDisplayEventHandler(
applicationObject_FolderContextMenuDisplay );
}

void applicationObject_FolderContextMenuDisplay(
Microsoft.Office.Core.CommandBar commandBar,
Microsoft.Office.Interop.Outlook.MAPIFolder folder )
{
CommandBarButton button = (CommandBarButton)commandBar.Controls.Add(
MsoControlType.msoControlButton, 1, Type.Missing,
commandBar.Controls.Count - 1, true );
// Exception occurs on next line:
button.Tag = folder.EntryID + ";" + folder.StoreID;
button.Caption = "My Command";
}

***** end code *****

The specific exception (e.Message) is:

The parameter is incorrect. (Exception from HRESULT: 0x80070057
(E_INVALIDARG))

 

[280Z28]

Well, it seems the string was too long (that's my guess). When I set it to
just folder.EntryID, it worked. Maybe the exception message could be made
more informative.