Using new api v2 for getting property groups is asking for .tag field

I am trying to get the property groups metadata for a file in my dropbox. The problem is the include_property_groups parameter. 

From the API example, I can't see any data with include_property_groups  :

curl -X POST https://api.dropboxapi.com/2/files/get_metadata \
    --header "Authorization: Bearer " \
    --header "Content-Type: application/json" \
    --data "{\"path\": \"/Homework/math\",\"include_media_info\": false,\"include_deleted\": false,\"include_has_explicit_shared_members\": false}"

But the data returned from the request has property_groups :

{
    ".tag": "folder",
    "name": "math",
    "id": "id:a4ayc_80_OEAAAAAAAAAXz",
    "path_lower": "/homework/math",
    "path_display": "/Homework/math",
    "sharing_info": {
        "read_only": false,
        "parent_shared_folder_id": "84528192421",
        "traverse_only": false,
        "no_access": false
    },
    "property_groups": [
        {
            "template_id": "ptid:1a5n2i6d3OYEAAAAAAAAAYa",
            "fields": [
                {
                    "name": "Security Policy",
                    "value": "Confidential"
                }
            ]
        }
    ]
}

If am using include_property_groups  in my API call, the response throws an error : "request body: include_property_groups: missing '.tag' key". I tried anything, for the '.tag' key, but I can't seem to get it work. 

Can you help ?

Thank you.