diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 5171fc9..bec74a9 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -601,8 +601,7 @@
  *                 #MBEDTLS_RSA_PUBLIC or #MBEDTLS_RSA_PRIVATE (deprecated).
  * \param ilen     The length of the plaintext in Bytes.
  * \param input    The input data to encrypt. This must be a readable
- *                 buffer of size \p ilen Bytes. It may be \c NULL if
- *                 `ilen == 0`.
+ *                 buffer of size \p ilen Bytes. This must not be \c NULL.
  * \param output   The output buffer. This must be a writable buffer
  *                 of length \c ctx->len Bytes. For example, \c 256 Bytes
  *                 for an 2048-bit RSA modulus.
@@ -642,8 +641,7 @@
  *                 #MBEDTLS_RSA_PUBLIC or #MBEDTLS_RSA_PRIVATE (deprecated).
  * \param ilen     The length of the plaintext in Bytes.
  * \param input    The input data to encrypt. This must be a readable
- *                 buffer of size \p ilen Bytes. It may be \c NULL if
- *                 `ilen == 0`.
+ *                 buffer of size \p ilen Bytes. This must not be \c NULL.
  * \param output   The output buffer. This must be a writable buffer
  *                 of length \c ctx->len Bytes. For example, \c 256 Bytes
  *                 for an 2048-bit RSA modulus.
@@ -687,8 +685,7 @@
  * \param label_len  The length of the label in Bytes.
  * \param ilen       The length of the plaintext buffer \p input in Bytes.
  * \param input      The input data to encrypt. This must be a readable
- *                   buffer of size \p ilen Bytes. It may be \c NULL if
- *                   `ilen == 0`.
+ *                   buffer of size \p ilen Bytes. This must not be \c NULL.
  * \param output     The output buffer. This must be a writable buffer
  *                   of length \c ctx->len Bytes. For example, \c 256 Bytes
  *                   for an 2048-bit RSA modulus.