diff --git a/includes/api/ApiFormatBase.php b/includes/api/ApiFormatBase.php
index 1baaaecc86e..7a08ed7d1bd 100644
--- a/includes/api/ApiFormatBase.php
+++ b/includes/api/ApiFormatBase.php
@@ -119,7 +119,7 @@ abstract class ApiFormatBase extends ApiBase {
 	 * Initialize the printer function and prepare the output headers.
 	 * @param bool $unused Always false since 1.25
 	 */
-	function initPrinter( $unused ) {
+	function initPrinter( $unused = false ) {
 		if ( $this->mDisabled ) {
 			return;
 		}