vb.net - Argument 'Expressions' cannot be converted into type "DBNull" -


i badly stuck error. when try call form (frmrecipe) menu.vb gives error on line trying maximum id recipe table, here code below

 function get_max_id() string         try             dim cmd new sqlcommand("select max(recipe_id) recipe", conn)             cmd.commandtype = commandtype.text             dim dr sqldatareader             dr = cmd.executereader             if dr.read                 return format(val(dr(0)).tostring + 1, "0000")             else                 return "0001"             end if         catch ex exception             msgbox(ex.message)         end try     end function

recipe_id type of varchar in database

try this:

return format(val(dr(0).tostring) + 1, "0000")

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