diff --git a/include/mbedtls/x509_csr.h b/include/mbedtls/x509_csr.h
index 0a069df..a0f1278 100644
--- a/include/mbedtls/x509_csr.h
+++ b/include/mbedtls/x509_csr.h
@@ -264,16 +264,13 @@
  * \param ctx       CSR to write away
  * \param buf       buffer to write to
  * \param size      size of the buffer
- * \param f_rng     RNG function (for signature, see note)
+ * \param f_rng     RNG function. This must not be \c NULL.
  * \param p_rng     RNG parameter
  *
  * \return          length of data written if successful, or a specific
  *                  error code
  *
- * \note            f_rng may be NULL if RSA is used for signature and the
- *                  signature is made offline (otherwise f_rng is desirable
- *                  for countermeasures against timing attacks).
- *                  ECDSA signatures always require a non-NULL f_rng.
+ * \note            \p f_rng is used for the signature operation.
  */
 int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, size_t size,
                        int (*f_rng)(void *, unsigned char *, size_t),
@@ -287,15 +284,12 @@
  * \param ctx       CSR to write away
  * \param buf       buffer to write to
  * \param size      size of the buffer
- * \param f_rng     RNG function (for signature, see note)
+ * \param f_rng     RNG function. This must not be \c NULL.
  * \param p_rng     RNG parameter
  *
  * \return          0 if successful, or a specific error code
  *
- * \note            f_rng may be NULL if RSA is used for signature and the
- *                  signature is made offline (otherwise f_rng is desirable
- *                  for countermeasures against timing attacks).
- *                  ECDSA signatures always require a non-NULL f_rng.
+ * \note            \p f_rng is used for the signature operation.
  */
 int mbedtls_x509write_csr_pem( mbedtls_x509write_csr *ctx, unsigned char *buf, size_t size,
                        int (*f_rng)(void *, unsigned char *, size_t),