diff --git a/ApiVisualEditor.php b/ApiVisualEditor.php
index b7c53302d5..f42b976240 100644
--- a/ApiVisualEditor.php
+++ b/ApiVisualEditor.php
@@ -669,7 +669,10 @@ class ApiVisualEditor extends ApiBase {
 	 * @return array
 	 */
 	public static function getAvailableNamespaceIds( Config $config ) {
-		$availableNamespaces = $config->get( 'VisualEditorAvailableNamespaces' );
+		$availableNamespaces =
+			// Note: existing numeric keys might exist, and so array_merge cannot be used
+			(array) $config->get( 'VisualEditorAvailableNamespaces' ) +
+			(array) ExtensionRegistry::getInstance()->getAttribute( 'VisualEditorAvailableNamespaces' );
 		return array_map( function ( $namespace ) {
 			// Convert canonical namespace names to IDs
 			return is_numeric( $namespace ) ?