Can not insert FeedItem through apexrest

When trying to create a FeedItem through the ApexRest I'm always getting the following error:

[{"errorCode":"APEX_ERROR","message":"System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Body]: [Body]\n\nClass.WS_SALES_V1_Document.post: line 61, column 1"}]

But according to the Salesforce docs, the Body field is Optional when Type is ContentPost.
Apex code of FeedItem creation(the insert fi; is line 61 by the way):

FeedItem fi = new FeedItem();
fi.ParentId = o.Id;
fi.Type = 'ContentPost';
fi.Visibility = 'AllUsers';
insert fi;

Is it a bug? Is documentation not up-to-date?

EDIT

I'm using API version 36.0