We're getting a lot of these, and my feeling is that the root cause is bad UTF-8 that we're allowing into the parser pipeline somehow (from an extension, from the database, from...?). That eventually causes a preg_* method to return null, and then that null propagates out and causes something somewhere else to fail. [Parser: enforce @return string types, and types of private methods (1247112) · Gerrit Code Review](https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1247112) attempts to move the enforcement boundary slightly earlier, in the process of fixing T415068: PHP Deprecated: substr_count(): Passing null to parameter #1 ($haystack) of type string is deprecated, which is another bug of the same sort. T414712: PHP Deprecated: preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated is also similar, and was fixed in Parser: handleMagicLinks should always return string (1235811) · Gerrit Code Review by a patch which avoided the null by allowed the bad UTF-8 to leak out of the method, where it will probably cause Yet Another of these "anomalous null" bugs.