Problem with one of my shows

 There's an error on one of my channels shows menu
"error must turn a value (21)"

I think it involves the method below but I don't know what it is

Function CreateHomeworkHelperzMenu() as integer

 screen = CreateObject("roGridScreen")

 port = CreateObject("roMessagePort")

 screen.SetMessagePort(port)

 

 'prepare the screen for display and get ready to begin

 screen=preShowPosterScreen("", "")

 if screen=invalid then

 print "unexpected error in preShowPosterScreen"

 return

 end if

 'set to go, time to get started

 showPosterScreen(screen)

End Function