diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 7b508c5..cfb66dd 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -118,7 +118,10 @@
  */
 typedef struct
 {
-    int ver;                    /*!<  Always 0.*/
+    int ver;                    /*!<  Reserved for internal purposes.
+                                 *    Do not set this field in application
+                                 *    code. Its meaning might change without
+                                 *    notice. */
     size_t len;                 /*!<  The size of \p N in Bytes. */
 
     mbedtls_mpi N;                      /*!<  The public modulus. */
@@ -148,6 +151,7 @@
                                      mask generating function used in the
                                      EME-OAEP and EMSA-PSS encodings. */
 #if defined(MBEDTLS_THREADING_C)
+    /* Invariant: the mutex is initialized iff ver != 0. */
     mbedtls_threading_mutex_t mutex;    /*!<  Thread-safety mutex. */
 #endif
 }