WPF WebBrowser execCommand("styleWithCSS")

I am trying to set some arbitrary css onto an element in wpf's WebBrowser

here's the code

Dim hd As IHTMLDocument2 = wb.Document
hd.execCommand("styleWithCSS", False, CObj("margin:0px"))

but i get an error

Trying to revoke a drop target that has not been registered (Exception from HRESULT: 0x80040100 (DRAGDROP_E_NOTREGISTERED))

other simple commands such as cut do work

please advise