diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index a5d0a34..f1d53bd 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -70,6 +70,9 @@
 typedef struct
 {
     pthread_mutex_t mutex;
+    /* is_valid is 0 after a failed init or a free, and nonzero after a
+     * successful init. This field is not considered part of the public
+     * API of Mbed TLS and may change without notice. */
     char is_valid;
 } mbedtls_threading_mutex_t;
 #endif