diff --git a/drivers/arm/css/scmi/scmi_private.h b/drivers/arm/css/scmi/scmi_private.h
index 61437f6..a684ca5 100644
--- a/drivers/arm/css/scmi/scmi_private.h
+++ b/drivers/arm/css/scmi/scmi_private.h
@@ -136,7 +136,7 @@
 	uint64_t res_b; /* Reserved */
 	uint32_t flags;
 	volatile uint32_t len;
-	uint32_t msg_header;
+	volatile uint32_t msg_header;
 	uint32_t payload[];
 } mailbox_mem_t;