Can I enable a web hook subscription through the REST api?

I see that a subscription is in status "onProbation". But whenever I try to update the subscription I get the following error:

{"$id": "1","innerException": null,"message": "No subscription could be found with id 6b7733ac-d062-44dd-a4ee-c8a449751b5b.","typeName": "Microsoft.VisualStudio.Services.ServiceHooks.WebApi.SubscriptionNotFoundException, Microsoft.VisualStudio.Services.ServiceHooks.WebApi, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a","typeKey": "SubscriptionNotFoundException","errorCode": 0,"eventId": 4501
}

I can post the same request as a GET message and it returns a subscription with the specified id.