Index: wp-admin/includes/template.php
===================================================================
--- wp-admin/includes/template.php	(revision 8454)
+++ wp-admin/includes/template.php	(working copy)
@@ -670,7 +670,7 @@
 	} else {
 		$edit = $user_object->user_login;
 	}
-	$role_name = translate_with_context($wp_roles->role_names[$role]);
+  $role_name = $wp_roles->role_names[$role] ? translate_with_context($wp_roles->role_names[$role]) : __('None');
 	$r = "<tr id='user-$user_object->ID'$style>
 		<th scope='row' class='check-column'><input type='checkbox' name='users[]' id='user_{$user_object->ID}' class='$role' value='{$user_object->ID}' /></th>
 		<td><strong>$edit</strong></td>