asp.net can't bind the datareader to a listbox..

I am using data access application block. the program runs fine untill it reached 'Listbox1.datasource = drcategories".
the error message is " "Object reference not set to an instance of an object." any idea why it does that?

Dim drCategories As SqlDataReader
drCategories = SqlHelper.ExecuteReader(sqlConn, CommandType.Text, "Select categoryid From Categories")
Try
ListBox1.DataSource = drCategories
Listbox1.databind()
Catch exp As Exception
Response.Write(exp.message)
End Try

resolved...