Inserting a function field in writer does nothing

Hello,

I’m sure this is a dumb question, but am stuck with it and can’t find hint to solve it

I try to insert a field in libreoffice writer (CTRL-F2 > Functions > Execute macro > MyDocument/Standard/Module1/DateCalculation)
But when clicking on “Insert”, there is only a grey space appearing in my text, and update fields command doesn’t do nothing.

Here is my macro:

Function DateCalculation as Integer
		monage=Int(DateDiff("d", "25/12/1827", Now())/365)
		MsgBox monage
		DateCalculation = monage
End Function

Is there something obvious I’m missing?

Many thanks,
Emmanuel