diff --git a/kernel/ethosu_mailbox.c b/kernel/ethosu_mailbox.c
index 014ceb8..b5cde0d 100644
--- a/kernel/ethosu_mailbox.c
+++ b/kernel/ethosu_mailbox.c
@@ -105,7 +105,8 @@
 
 static size_t ethosu_queue_capacity(struct ethosu_core_queue *queue)
 {
-	return queue->header.size - ethosu_queue_available(queue);
+	return max_t(ssize_t,
+		     queue->header.size - ethosu_queue_available(queue) - 1, 0);
 }
 
 static int ethosu_queue_write(struct ethosu_mailbox *mbox,