diff --git a/src/plugins/imapsieve/imap-sieve-storage.c b/src/plugins/imapsieve/imap-sieve-storage.c
index 4a76aa69757120a94c4e207196082bd6f9b14762..d9a849df1ba34c452f5f1d10b86fb2f54df9efa0 100644
--- a/src/plugins/imapsieve/imap-sieve-storage.c
+++ b/src/plugins/imapsieve/imap-sieve-storage.c
@@ -578,6 +578,7 @@ imap_sieve_mailbox_transaction_run(
 		break;
 	case IMAP_SIEVE_CMD_COPY:
 	case IMAP_SIEVE_CMD_MOVE:
+		i_assert(src_box != NULL);
 		cause = "COPY";
 		can_discard = TRUE;
 		break;