Viewing API Call metrics by API key not working?

In the metrics view, I see an option to view API calls by users, either by entering their email or their API Key. When I entered the API key I see a user sending, nothing shows up in the API calls graph, though I can see that they're using that key in the request headers. Is this a bug on readme's system or is there something I'm doing wrong? It would be very helpful to view calls by API Key given that many of our users arent logged in with email addresses.