Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 20561)
+++ wp-includes/pluggable.php	(working copy)
@@ -1647,7 +1647,7 @@
 	elseif ( !empty($email) && 'gravatar_default' == $default )
 		$default = '';
 	elseif ( 'gravatar_default' == $default )
-		$default = "$host/avatar/s={$size}";
+		$default = "$host/avatar/?s={$size}";
 	elseif ( empty($email) )
 		$default = "$host/avatar/?d=$default&s={$size}";
 	elseif ( strpos($default, 'http://') === 0 )