Need to get the previous value in Sharepoint library

i am taking the Item Updating as well as updated event. i need to get the previous value of a people picker field. i tried before properties, listitem.. always getting null values.. i tried display name, internal name.. no success..

but according to this post- http://www.synergyonline.com/Blog/Lists/Posts/Post.aspx?ID=122, it should not be empty, right?
please guide me

string str= Convert.ToString(properties.BeforeProperties[internal name of my people picker field]);