Item search by custom field

I have only one deal with custom field = “xxxxx” .I’m trying to get this deal.
I’m trying to use the /itemSearch/field endpoint, but what i’m getting is the 404 HTTP response.
This is the URL i’m using:
itemSearch/field?term=xxxxx
&field_type=dealField
&field_key=yyyyyy
&api_token=zzzzzz
I’m making something wrong or this response means that there is no Deal with the ‘‘xxxxx’’ custom field?