Null Vallue Lookup Column Issue

I've tried to create a SharePoint view with a lookup column value equals null, but even though there are items with "Phase" column equal to NULL, nothing is shown in the view.

I looked up the column value through the code, and it is NULL; I looked up the view CAML and it looks right: "<Query><Where><IsNull><FieldRef Name="Phase" /></IsNull></Where></Query>".

I really have no more ideas why it behaves this way.

Any suggestions are welcome