Conversion from type DBNull to type String is not valid VB.Net

in a search page in my asp.net+vb web page.  iam using a code as under but when the field is balnk it gives error.

statustxt.Text = dt.Tables("PERS").Rows(0).Item("OutDetails")

Then i tried this code also 

If dt.Tables("PERS").Rows(0).Item("Courser") Is System.DBNull.Value <> True Then statustxt.Text = dt.Tables("PERS").Rows(0).Item("OutDetails") Else statustxt.Text = "" End If

but it also gives error as 

Conversion from type 'DBNull' to type 'String' is not valid.