diff --git a/includes/model/Shifts_model.php b/includes/model/Shifts_model.php
index 43ff0c91..11309e11 100644
--- a/includes/model/Shifts_model.php
+++ b/includes/model/Shifts_model.php
@@ -39,13 +39,13 @@ function Shift_update($shift) {
  * Update a shift by its external id.
  */
 function Shift_update_by_psid($shift) {
-  return sql_query("UPDATE `Shifts` SET
-      `start`=" . sql_escape($shift['start']) . ",
-      `end`=" . sql_escape($shift['end']) . ",
-      `RID`=" . sql_escape($shift['RID']) . ",
-      `name`=" . sql_null($shift['name']) . ",
-      `URL`=" . sql_null($shift['URL']) . "
-      WHERE `PSID`=" . sql_escape($shift['PSID']));
+  $shift_source = sql_select("SELECT `SID` FROM `Shifts` WHERE `PSID`=" . $shift['PSID']);
+  if ($shift_source === false)
+    return false;
+  if (count($shift_source) == 0)
+    return null;
+  $shift['SID'] = $shift_source['SID'];
+  return Shift_update($shift);
 }
 
 /**