Crystal Reports Pb SUM function

Message

   Hello everyone !!

 I try to use CR in VS 2008 (VB .net) and I've got a problem while using SUM in formula Fields.

 I would like to make the SUM of one field (montant) in a table (courses) but all it says when I save is "there is an error in this formula". How can I do to get the SUM of my field please ??

 Thanks

 Ps : I've try in the formula workshop :

code1

Sum ({COURSES.MONTANT})

code 2

Sum ({COURSES.MONTANT});