diff --git a/kernel/ethosu_mailbox.c b/kernel/ethosu_mailbox.c
index 3e7284b..5b105d8 100644
--- a/kernel/ethosu_mailbox.c
+++ b/kernel/ethosu_mailbox.c
@@ -31,6 +31,7 @@
 #include <linux/jiffies.h>
 #include <linux/resource.h>
 #include <linux/uio.h>
+#include <linux/bug.h>
 
 /****************************************************************************
  * Includes
@@ -65,6 +66,7 @@
 int ethosu_mailbox_register(struct ethosu_mailbox *mbox,
 			    struct ethosu_mailbox_msg *msg)
 {
+	WARN_ON_ONCE(!mutex_is_locked(&mbox->dev->mutex));
 	msg->id = idr_alloc_cyclic(&mbox->msg_idr, msg, 0, INT_MAX, GFP_KERNEL);
 	if (msg->id < 0)
 		return msg->id;
@@ -75,6 +77,7 @@
 void ethosu_mailbox_deregister(struct ethosu_mailbox *mbox,
 			       struct ethosu_mailbox_msg *msg)
 {
+	WARN_ON_ONCE(!mutex_is_locked(&mbox->dev->mutex));
 	idr_remove(&mbox->msg_idr, msg->id);
 }
 
@@ -82,8 +85,10 @@
 					       int msg_id,
 					       uint32_t msg_type)
 {
-	struct ethosu_mailbox_msg *ptr = (struct ethosu_mailbox_msg *)idr_find(
-		&mbox->msg_idr, msg_id);
+	struct ethosu_mailbox_msg *ptr;
+
+	WARN_ON_ONCE(!mutex_is_locked(&mbox->dev->mutex));
+	ptr = (struct ethosu_mailbox_msg *)idr_find(&mbox->msg_idr, msg_id);
 
 	if (ptr == NULL)
 		return ERR_PTR(-ENOENT);
@@ -99,6 +104,7 @@
 	struct ethosu_mailbox_msg *cur;
 	int id;
 
+	WARN_ON_ONCE(!mutex_is_locked(&mbox->dev->mutex));
 	idr_for_each_entry(&mbox->msg_idr, cur, id) {
 		cur->fail(cur);
 	}