Codecov API "compare" not returning diff totals

Description

I’m trying to build out a reporting app which needs to fetch incremental coverage data for our repo. That is, for the range of commits BASE…HEAD, we’d like to know what percent of added lines were tested. This appears to work fine in the web UI for some ranges; I can see results here, for example. When I try this on other (especially larger) ranges like this one, I get “codecov . io unexpectedly closed the connection.” Added to that, for any range of commits, the response from the compare API endpoint at /api/gh/:owner/:repo/compare/:base...:head always has "diff": null even though there’s clearly a difference between the commits.

Is this endpoint broken? If so, is there a workaround or simple way to compute the incremental coverage?

Repository

GitHub: ampproject/amphtml

Versions

Browser: Any

Additional Information

N/A