Error when I Get or Post custom fields

I can use standard fields like 'type' and 'title' (e.g. "?fields=type,title") but when I try and restrict GETs to custom fields as well (e.g. "?fields=type,title,tt") I get the following error:

"Invalid query provided, double check that the fields and parameters you defined are correct and exist. SQLSTATE[42S22]: Column not found: 1054 Unknown column 't.tt' in 'field list'"
]

I have tried using "?fields=type,title,field_tt" and that throws the same eror.

Likewise I can post to the title field but and data posted to other fields is not created, only the title field.

EDIT: Just realised that I wasn't using the /entity_node/ in the url - it now works.