Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 44358)
+++ wp-includes/user.php	(working copy)
@@ -2719,7 +2719,7 @@
 			return;
 		}
 
-		if ( $wpdb->get_var( $wpdb->prepare( "SELECT user_email FROM {$wpdb->users} WHERE user_email=%s", $_POST['email'] ) ) ) {
+		if ( email_exists( $_POST['email'] ) )  {
 			$errors->add(
 				'user_email',
 				__( '<strong>ERROR</strong>: The email address is already used.' ),