WEB API - Error with Custom Lookup to Custom Entity

Hi,

I'm trying to use the new web api to import a record. I have the details in json file. When I exclude the lookup field the record goes in fine.

When I include the lookup field I get a 400 error: Bad Request.

I've tried multiple ways to format this and believe this is the correct way:

 "wt_propertyid@odata.bind": "mycrm.crm4.dynamics.com/.../wt_properties(31095689-ACDC-E611-8110-3863BB355D40)"

or

 "wt_propertyid@odata.bind": "/wt_properties(31095689-ACDC-E611-8110-3863BB355D40)"

The details are:

Parent Entity (Property) wt_property

Child Entity (Space) wt_space

Lookup to Parent wt_property

I have tried with and without the id in the field name (before the '@')