cannot add usernames in a dropdownlist .

Hi, im trying to show usernames in a dropdownlist , but only the ones who are in an admin roles that i have defined.
the code i got is this:
Dim AdminUsers As String() = Roles.GetUsersInRole("admin")
For Each usr As MembershipUser In AdminUsers
DropDownList1.Items.Add(usr.UserName)
Next
this is the error:
value of type 'string' cannot be converted to 'system.web.security.membershipuser'

any help would be appreciate it. 
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