Virtual Field mapping Multi-value field problem

I have a web form monitored by a virtual field activitie.The activitie maps two number fields and one of them is a multi-value field, to a table in a Oracle database that have 2 Foreign Key fields .

The activitie works fine if the multi-value field is populated, but, when the user let the field empty, Decs raises a Foreign Key error and Domino presents a error message on the browser.

When I consult the Oracle by SQL Plus, the rows corresponding to my document are gone.

Why domino raises a error?

Are there any special configuration that I have to make on the virtual field activitie?

Does any body suffer this before?