diff --git a/include/mbedtls/aead_chacha20_poly1305.h b/include/mbedtls/aead_chacha20_poly1305.h
index a1ccf31..6c8e420 100644
--- a/include/mbedtls/aead_chacha20_poly1305.h
+++ b/include/mbedtls/aead_chacha20_poly1305.h
@@ -124,6 +124,7 @@
  * \param aad_len       The length (in bytes) of the AAD. The length has no
  *                      restrictions.
  * \param aad           Buffer containing the AAD.
+ *                      This pointer can be NULL if aad_len == 0.
  *
  * \return              MBEDTLS_ERR_AEAD_CHACHA20_POLY1305_BAD_INPUT_DATA is returned
  *                      if \p ctx or \p aad are NULL.
@@ -151,7 +152,9 @@
  * \param ctx           The ChaCha20-Poly1305 context.
  * \param len           The length (in bytes) of the data to encrypt or decrypt.
  * \param input         Buffer containing the data to encrypt or decrypt.
+ *                      This pointer can be NULL if len == 0.
  * \param output        Buffer to where the encrypted or decrypted data is written.
+ *                      This pointer can be NULL if len == 0.
  *
  * \return              MBEDTLS_ERR_AEAD_CHACHA20_POLY1305_BAD_INPUT_DATA is returned
  *                      if \p ctx, \p input, or \p output are NULL.
@@ -195,9 +198,12 @@
  *                      parameter does not matter.
  * \param aad_len       The length (in bytes) of the AAD data to process.
  * \param aad           Buffer containing the additional authenticated data (AAD).
+ *                      This pointer can be NULL if aad_len == 0.
  * \param ilen          The length (in bytes) of the data to encrypt or decrypt.
  * \param input         Buffer containing the data to encrypt or decrypt.
+ *                      This pointer can be NULL if ilen == 0.
  * \param output        Buffer to where the encrypted or decrypted data is written.
+ *                      This pointer can be NULL if ilen == 0.
  * \param mac           Buffer to where the computed 128-bit (16 bytes) MAC is written.
  *
  * \return              MBEDTLS_ERR_AEAD_CHACHA20_POLY1305_BAD_INPUT_DATA is returned
diff --git a/include/mbedtls/chacha20.h b/include/mbedtls/chacha20.h
index ab87f66..ccce122 100644
--- a/include/mbedtls/chacha20.h
+++ b/include/mbedtls/chacha20.h
@@ -136,7 +136,9 @@
  * \param ctx       The ChaCha20 context.
  * \param size      The length (in bytes) to process. This can have any length.
  * \param input     Buffer containing the input data.
+ *                  This pointer can be NULL if size == 0.
  * \param output    Buffer containing the output data.
+ *                  This pointer can be NULL if size == 0.
  *
  * \return          MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA if the ctx, input, or
  *                  output pointers are NULL.
diff --git a/include/mbedtls/poly1305.h b/include/mbedtls/poly1305.h
index 1aa55ae..ea9364a 100644
--- a/include/mbedtls/poly1305.h
+++ b/include/mbedtls/poly1305.h
@@ -85,6 +85,7 @@
  * \param ctx           The Poly1305 context.
  * \param ilen          The input length (in bytes). Any value is accepted.
  * \param input         Buffer containing the input data to Process.
+ *                      This pointer can be NULL if ilen == 0.
  *
  * \return              MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA is returned if ctx
  *                      or input are NULL.