I have item content in the main namespace, which seems to be the problem here.
tests should be more robust and be able to handle or work around this.
Fatal error: Uncaught exception 'MWContentSerializationException' with message 'Failed to decode as ' in /var/www/wiki/core/extensions/Wikidata/extensions/Wikibase/lib/includes/store/EntityContentDataCodec.php:201
Stack trace:
#0 /var/www/wiki/core/extensions/Wikidata/extensions/Wikibase/lib/includes/store/EntityContentDataCodec.php(250): Wikibase\Lib\Store\EntityContentDataCodec->decodeEntityContentData()
#1 /var/www/wiki/core/extensions/Wikidata/extensions/Wikibase/repo/includes/content/EntityHandler.php(394): Wikibase\Lib\Store\EntityContentDataCodec->decodeRedirect()
#2 /var/www/wiki/core/includes/content/ContentHandler.php(156): Wikibase\Repo\Content\EntityHandler->unserializeContent()
#3 /var/www/wiki/core/tests/phpunit/MediaWikiTestCase.php(440): ContentHandler::makeContent()
#4 /var/www/wiki/core/tests/phpunit/includes/PrefixSearchTest.php(9): MediaWikiTestCase->insertPage()
#5 /var/www/wiki/core/tests/phpunit/MediaWikiTestCase.php(127): PrefixSearchTest->addDBData()
#6 /var/www/wiki/core/vendor/phpunit/phpunit/src/Framework/TestSuite.php(722): MediaWikiTestCase->run()
#7 /var/www/wiki/core/vendor/phpunit/phpunit/src/Framework/TestSuite.php(722): PHPUnit_Framework_TestSuite->run()
#8 /var/www/wiki/core/vendor/phpunit/phpunit/src/Framework/TestSuite.php(722): PHPUnit_Framework_TestSuite->run()
#9 /var/www/wiki/core/vendor/phpunit/phpunit/src/TextUI/TestRunner.php(406): PHPUnit_Framework_TestSuite->run()
#10 /var/www/wiki/core/vendor/phpunit/phpunit/src/TextUI/Command.php(153): PHPUnit_TextUI_TestRunner->doRun()
#11 /var/www/wiki/core/vendor/phpunit/phpunit/src/TextUI/Command.php(105): PHPUnit_TextUI_Command->run()
#12 /var/www/wiki/core/tests/phpunit/phpunit.php(241): PHPUnit_TextUI_Command::main()
#13 {main}
[50c1446e] [no req] MWContentSerializationException from line 201 of /var/www/wiki/core/extensions/Wikidata/extensions/Wikibase/lib/includes/store/EntityContentDataCodec.php: Failed to decode as
Backtrace:
#0 /var/www/wiki/core/extensions/Wikidata/extensions/Wikibase/lib/includes/store/EntityContentDataCodec.php(250): Wikibase\Lib\Store\EntityContentDataCodec->decodeEntityContentData(string, NULL)
#1 /var/www/wiki/core/extensions/Wikidata/extensions/Wikibase/repo/includes/content/EntityHandler.php(394): Wikibase\Lib\Store\EntityContentDataCodec->decodeRedirect(string, NULL)
#2 /var/www/wiki/core/includes/content/ContentHandler.php(156): Wikibase\Repo\Content\EntityHandler->unserializeContent(string, NULL)
#3 /var/www/wiki/core/tests/phpunit/MediaWikiTestCase.php(440): ContentHandler::makeContent(string, Title)
#4 /var/www/wiki/core/tests/phpunit/includes/PrefixSearchTest.php(9): MediaWikiTestCase->insertPage(string)
#5 /var/www/wiki/core/tests/phpunit/MediaWikiTestCase.php(127): PrefixSearchTest->addDBData()
#6 /var/www/wiki/core/vendor/phpunit/phpunit/src/Framework/TestSuite.php(722): MediaWikiTestCase->run(PHPUnit_Framework_TestResult)
#7 /var/www/wiki/core/vendor/phpunit/phpunit/src/Framework/TestSuite.php(722): PHPUnit_Framework_TestSuite->run(PHPUnit_Framework_TestResult)
#8 /var/www/wiki/core/vendor/phpunit/phpunit/src/Framework/TestSuite.php(722): PHPUnit_Framework_TestSuite->run(PHPUnit_Framework_TestResult)
#9 /var/www/wiki/core/vendor/phpunit/phpunit/src/TextUI/TestRunner.php(406): PHPUnit_Framework_TestSuite->run(PHPUnit_Framework_TestResult)
#10 /var/www/wiki/core/vendor/phpunit/phpunit/src/TextUI/Command.php(153): PHPUnit_TextUI_TestRunner->doRun(PHPUnit_Framework_TestSuite, array)
#11 /var/www/wiki/core/vendor/phpunit/phpunit/src/TextUI/Command.php(105): PHPUnit_TextUI_Command->run(array, boolean)
#12 /var/www/wiki/core/tests/phpunit/phpunit.php(241): PHPUnit_TextUI_Command::main()