diff --git a/inc/vmapi/hf/call.h b/inc/vmapi/hf/call.h
index 7de8160..7f229d2 100644
--- a/inc/vmapi/hf/call.h
+++ b/inc/vmapi/hf/call.h
@@ -93,6 +93,8 @@
  * - 0, if the caller is a secondary VM
  * - the ID of the vCPU to run to receive the message, if the caller is the
  * primary VM.
+ * - HF_INVALID_VCPU if the caller is the primary VM and no vCPUs on the target
+ * VM are currently waiting to receive a message.
  */
 static inline int64_t hf_mailbox_send(uint32_t vm_id, size_t size)
 {