diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 4ff5bdd..489f2ed 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -601,7 +601,8 @@
  *                 #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. This must not be \c NULL.
+ *                 buffer of size \p ilen Bytes. It may be \c NULL if
+ *                 `ilen == 0`.
  * \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.
@@ -641,7 +642,8 @@
  *                 #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. This must not be \c NULL.
+ *                 buffer of size \p ilen Bytes. It may be \c NULL if
+ *                 `ilen == 0`.
  * \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.
@@ -685,7 +687,8 @@
  * \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. This must not be \c NULL.
+ *                   buffer of size \p ilen Bytes. It may be \c NULL if
+ *                   `ilen == 0`.
  * \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.