remote: sendmail: fatal: git(1504): No recipient addresses found in message header

Earlier today when pushing onto git-rw.tpo, got this error (the push worked though):

remote: via /srv/git.torproject.org/git-helpers/post-receive-diff
remote: == 00-sync-to-mirror == 
remote: == commit-mail == 
remote: sendmail: fatal: git(1504): No recipient addresses found in message header
remote: sendmail: fatal: git(1504): No recipient addresses found in message header
remote: sendmail: fatal: git(1504): No recipient addresses found in message header
remote: sendmail: fatal: git(1504): No recipient addresses found in message header
[...]