diff --git a/src/lib-sieve/sieve-storage-sync.c b/src/lib-sieve/sieve-storage-sync.c
index f6a93f00bc7d145a310a3f91ee1569d917e63f12..8af9aff4d31366f5ea9d08f62a0b31497fd77153 100644
--- a/src/lib-sieve/sieve-storage-sync.c
+++ b/src/lib-sieve/sieve-storage-sync.c
@@ -69,6 +69,7 @@ static int sieve_storage_sync_transaction_begin
 		sieve_storage_sys_warning(storage, "sync: "
 			"Failed to open user INBOX for attribute modifications: %s",
 			mailbox_get_last_error(inbox, &error));
+		mailbox_free(&inbox);
 		return -1;
 	}