Update a field to have its value null

Hello,

I'd like to know how I can update a User Table to have a field value set to null.

I tried

oTable.UserFields.Fields.Item("U_MyField").Value = nothing

oTable.UserFields.Fields.Item("U_MyFied").Value = System.DBNull.Value

oTable.UserFields.Fields.Item("U_MyFied").Value = Null

but none of them works

my field can be numeric, string, date, I need to update it to be sure it's null (not 0 or "" or 1900-01-01)

Thanks for your help

Sébastien