Requesting http://www.mediawiki.test/w/api.php?action=query&prop=revisions&format=jsonfm&rvlimit=1&titles=Bar&rvcontinue=2 from my test wiki yields this error message:
Notice: Undefined index: start in /home/ki/Projects/mediawiki/core/includes/api/ApiQueryRevisions.php on line 282
Fixing this seems trivial, so I will push a patch set to Gerrit very shortly.
Version: 1.21.x
Severity: minor