How to use Word'centimeterstopoints?(50分)

This is how i invoke word and use centimeterstopoints.
I want to change the page size of word document.
function ...
var
MyWord:variant;
begin
MyWord.create...
MyWord.documents.add(...
//--<when I use the statement:
MyWord.activedocument.pagesetup.RightMargin := MyWord.CentimetersToPoints(3.17);
//>--error pop out : unknown error.
MyWord.activedocument.close
Myword.quit;
end;

Is here anybody can tell me why I could not use the centimeterstopoints(3.17)?
and how to use it correctly?
Many Thanks.