Index: wp-includes/class-wp-image-editor-imagick.php
===================================================================
--- wp-includes/class-wp-image-editor-imagick.php	(revision 22097)
+++ wp-includes/class-wp-image-editor-imagick.php	(working copy)
@@ -143,7 +143,7 @@
 	 * @param string $mime_type
 	 * @return boolean
 	 */
-	public static function supports_mime_type( $mime_type = null ) {
+	public static function supports_mime_type( $mime_type ) {
 		if ( ! $mime_type )
 			return false;