When generating a list of the custom groups that utilize a particular sub-type, the sub-type was not properly escaped.
CiviCRM versions 5.3.0 and 4.6.37 (and earlier)
CiviCRM version 5.3.1 and 4.6.38 (and later)
Upgrade to the latest version of CiviCRM
Patrick Figel of Greenpeace for reporting the issue.
Seamus Lee of Australian greens for fixing the issue.