diff --git a/src/lib-sieve/plugins/vacation/cmd-vacation.c b/src/lib-sieve/plugins/vacation/cmd-vacation.c
index 57d2d9ab8482c076f5d2e12b97e39dcc22b05bd3..de31cff882c5b5a22debd45be4a271e1fb904c7d 100644
--- a/src/lib-sieve/plugins/vacation/cmd-vacation.c
+++ b/src/lib-sieve/plugins/vacation/cmd-vacation.c
@@ -961,7 +961,7 @@ static int act_vacation_send
 	/* Compose proper in-reply-to and references headers */
 
 	if ( mail_get_headers
-		(msgdata->mail, "references", &headers) ) {
+		(msgdata->mail, "references", &headers) < 0 ) {
 		return sieve_result_mail_error(aenv, msgdata->mail,
 			"vacation action: "
 			"failed to read header field `references'");