diff --git a/includes/SetupDynamicConfig.php b/includes/SetupDynamicConfig.php
index 800874253a1..e585cd6071e 100644
--- a/includes/SetupDynamicConfig.php
+++ b/includes/SetupDynamicConfig.php
@@ -244,11 +244,6 @@ $wgDefaultUserOptions['timecorrection'] = "System|$wgLocalTZoffset";
  */
 $wgCanonicalNamespaceNames = NamespaceInfo::CANONICAL_NAMES;
 
-// @todo UGLY UGLY
-if ( is_array( $wgExtraNamespaces ) ) {
-	$wgCanonicalNamespaceNames += $wgExtraNamespaces;
-}
-
 // Hard-deprecate setting $wgDummyLanguageCodes in LocalSettings.php
 if ( count( $wgDummyLanguageCodes ) !== 0 ) {
 	$wgSettings->warning(
diff --git a/tests/phpunit/unit/includes/SetupDynamicConfigTest.php b/tests/phpunit/unit/includes/SetupDynamicConfigTest.php
index 8951dedc670..e604207edd1 100644
--- a/tests/phpunit/unit/includes/SetupDynamicConfigTest.php
+++ b/tests/phpunit/unit/includes/SetupDynamicConfigTest.php
@@ -717,9 +717,9 @@ class SetupDynamicConfigTest extends MediaWikiUnitTestCase {
 			[ 'CanonicalNamespaceNames' => [ NS_MAIN => 'abc' ] ],
 			[],
 		];
-		yield '$wgExtraNamespaces set' => [
+		yield 'Setting $wgExtraNamespaces does not affect $wgCanonicalNamespaceNames' => [
 			[ 'ExtraNamespaces' => [ 100 => 'Extra' ] ],
-			[ 'CanonicalNamespaceNames' => NamespaceInfo::CANONICAL_NAMES + [ 100 => 'Extra' ] ],
+			[],
 		];
 		yield '$wgDummyLanguageCodes set' => [
 			[ 'DummyLanguageCodes' => [ 'qqq' => 'qqqq', 'foo' => 'bar', 'bh' => 'hb' ] ],