That's probably a bug I created when I tweaked the API to handle explicit field requests properly. I'll see what I can do to fix it today or tomorrow. *However* if you're using `fields=` and are only making use of a few fields *you are inadvertently hammering our servers for no good reason*. The `fields` modifier is there so you can request *specific fields*, not simply grab everything. If you use it in this way, your request will work as you need: https://atlas.ripe.net/api/v1/measurement/1011817/?fields=probes That'll return to you a list of all the probes used in measurement #1011817. If you want the probes and the `msm_id`, then just do this: https://atlas.ripe.net/api/v1/measurement/1011817/?fields=probes,msm_id ...repeat until you have *exactly* what you need and no more. You limit your bandwith consumption and the load on our servers. More importantly, the `fields=` trick is a hack that may not stick around for performance issues, so it's not a good idea to standardise on it. With that said though, as it was working this way before I broke it, I'll do what I can to fix it now :-)