diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 278fbbb..d8ef5d0 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -214,6 +214,13 @@
     void *p_entropy;            /*!< The context for the entropy function. */
 
 #if defined(MBEDTLS_THREADING_C)
+    /* Invariant: the mutex is initialized if and only if f_entropy != NULL.
+     * This means that the mutex is initialized during the initial seeding
+     * in mbedtls_ctr_drbg_seed() and freed in mbedtls_ctr_drbg_free().
+     *
+     * Note that this invariant may change without notice. Do not rely on it
+     * and do not access the mutex directly in application code.
+     */
     mbedtls_threading_mutex_t mutex;
 #endif
 }