diff --git a/htdocs/tz.php b/htdocs/tz.php
index b985b20f..8cbb4f28 100644
--- a/htdocs/tz.php
+++ b/htdocs/tz.php
@@ -26,7 +26,7 @@ $request = new CalDAVRequest();
 
 $code_file = sprintf( 'tz/%s.php', $action );
 if ( ! @include_once( $code_file ) ) {
-  $request->PreconditionFailed(400, "supported-action", 'The action "'.$action.'" is not understood.' );
+  $request->PreconditionFailed(400, "supported-action", 'The action "'.$action.'" is not understood.', 'urn:ietf:params:xml:ns:timezone-service' );
 }
 
 $request->DoResponse( 500, translate("The application failed to understand that request.") );
diff --git a/inc/tz/expand.php b/inc/tz/expand.php
index 89a8c215..9a068132 100644
--- a/inc/tz/expand.php
+++ b/inc/tz/expand.php
@@ -14,7 +14,7 @@ require_once('RRule-v2.php');
 
 if ( empty($format) ) $format = 'text/calendar';
 if ( $format != 'text/calendar' ) {
-  $request->PreconditionFailed(403, 'supported-format', 'This server currently only supports text/calendar format.');
+  $request->PreconditionFailed(403, 'supported-format', 'This server currently only supports text/calendar format.', 'urn:ietf:params:xml:ns:timezone-service' );
 }
 
 if ( empty($start) ) $start = sprintf( '%04d-01-01', date('Y'));
diff --git a/inc/tz/get.php b/inc/tz/get.php
index 7f3c05aa..e7726f91 100644
--- a/inc/tz/get.php
+++ b/inc/tz/get.php
@@ -13,7 +13,7 @@ require_once('vCalendar.php');
 
 if ( empty($format) ) $format = 'text/calendar';
 if ( $format != 'text/calendar' ) {
-  $request->PreconditionFailed(403, 'supported-format', 'This server currently only supports text/calendar format.');
+  $request->PreconditionFailed(403, 'supported-format', 'This server currently only supports text/calendar format.', 'urn:ietf:params:xml:ns:timezone-service');
 }
 
 $sql = 'SELECT our_tzno, tzid, active, olson_name, vtimezone, etag, ';