Productivity Analytics: API responds with error when called with multiple labels

When calling the productivity_analytics endpoint with multiple label_name[] it throws an error, see the following example:

https://gitlab.com/-/analytics/productivity_analytics.json?group_id=gitlab-org&project_id=gitlab-org%2Fgitlab&label_name[]=frontend&label_name[]=backend&merged_at_after=2019-10-04T00:00:00Z&merged_at_before=2019-12-03T23:59:59Z&chart_type=scatterplot&metric_type=days_to_merge

In contrast, with a single label_name[] the call succeeds:

https://gitlab.com/-/analytics/productivity_analytics.json?group_id=gitlab-org&project_id=gitlab-org%2Fgitlab&label_name[]=frontend&merged_at_after=2019-10-04T00:00:00Z&merged_at_before=2019-12-03T23:59:59Z&chart_type=scatterplot&metric_type=days_to_merge