Runtime error 94 : invalid use of null

[muzi1980]

When i run my form getnextorderno i get the following error : runtime error 94 invalid use of null.The code is below
Function GetNextOrderNo()
Dim nNextOrderNo As Double

DoCmd.GoToControl "[NextOrderNo]"
DoCmd.GoToRecord , , acNewRec



SendKeys ("%{DOWN}")
SendKeys ("{DOWN}")
DoCmd.RunMacro ("Maint Order.SetNewOrderNo")

nNextOrderNo = Val(Forms![Maint Order]![NextOrderNo].Column(0))


Forms![Maint Order]![Order_No] = nNextOrderNo
DoCmd.GoToControl "[Delivery_Date]"

End Function

[Moe1950]

What line of code gives you the error?

[GolferGuy]

And what is happening in this macro? "Maint Order.SetNewOrderNo"
It would be real good to see that macro code.