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

Question

Dear All,

I'm getting this error when reach this lines of my code
Conversion from type 'DBNull' to type 'String' is not valid

this is my code :

If DS.Tables(0).Rows.Count <> 0 Then
            blnD = DS.Tables(0).Rows(0)("ItemExist")
            FORM1_ID = DS.Tables(0).Rows(0)("FORMID")
           Prev_FORM_ID = DS.Tables(0).Rows(0)("PREVFORMID") <--- in this line the debuger will stop and give me the error :(

           lblDONE.Text = "Successfully Saved"

what to do ??