Thanks to everyone for their replies. I figured out on the nature of
my error. For my own subroutines I always call them using the call
keyword, in which parenthesis are required. I pretty much use message
boxes only for debugging purposes, and with 1 parameter () are
ignored. A little experimentation shows that 2 or more parameters with
() as I have been doing leads to problems.

My way to think about it now - msgbox( , , ..) is a noun that must be
embedded in a bigger context to make sense, but in msgbox "hello
world" msgbox is now functioning as a verb.

In my defense - when I type msgbox in the VBA editor the IntelliSense
which pops up suggests (), and sinse my code was compiling no-problem
I didn't realize I was being ungrammatical. It is strange how you can
misunderstand a basic feature of a language but nevertheless get by
with your own idiosyncratic work-arounds.

sub thanks()
MsgBox Prompt := "Now I understand", Title:= "Thanks again"
end sub

-John Coleman