Retreiving only portal items that don't have categories

[BorjaParés_Fuente]

Hi all,

I would like to query via REST items that don't have any category assigned.

I've tried with several options in the categroies parameter with any luck:

https://portal_machine/portal/sharing/rest/search?f=pjson&q=owner:user1&categories=''

https://portal_machine/portal/sharing/rest/search?f=pjson&q=owner:user1&categories=[]

https://portal_machine/portal/sharing/rest/search?f=pjson&q=owner:user1&categories=[""]

...

Any one knows how to get only no categories items?

Thanks in advance!

BR

Borja