diff --git a/include/mbedtls/chachapoly.h b/include/mbedtls/chachapoly.h
index 7de6f4e..80d20e5 100644
--- a/include/mbedtls/chachapoly.h
+++ b/include/mbedtls/chachapoly.h
@@ -115,27 +115,29 @@
  *                  all previous outputs of \c mbedtls_chachapoly_update(),
  *                  otherwise you can now safely use the plaintext.
  *
- * \param ctx       The ChachaPoly context to initialize.
+ * \param ctx       The ChachaPoly context to initialize. Must not be \c NULL.
  */
 void mbedtls_chachapoly_init( mbedtls_chachapoly_context *ctx );
 
 /**
- * \brief           This function releases and clears the specified ChaCha20-Poly1305 context.
+ * \brief           This function releases and clears the specified
+ *                  ChaCha20-Poly1305 context.
  *
- * \param ctx       The ChachaPoly context to clear.
+ * \param ctx       The ChachaPoly context to clear. May be \c NULL, in which
+ *                  case this function is a no-op.
  */
 void mbedtls_chachapoly_free( mbedtls_chachapoly_context *ctx );
 
 /**
- * \brief           This function sets the ChaCha20-Poly1305 symmetric encryption key.
+ * \brief           This function sets the ChaCha20-Poly1305
+ *                  symmetric encryption key.
  *
  * \param ctx       The ChaCha20-Poly1305 context to which the key should be
- *                  bound.
+ *                  bound. Must be initialized.
  * \param key       The 256-bit (32 bytes) key.
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA
- *                  if \p ctx or \p key are NULL.
+ * \return          A negative error code on failure.
  */
 int mbedtls_chachapoly_setkey( mbedtls_chachapoly_context *ctx,
                                const unsigned char key[32] );
@@ -155,14 +157,13 @@
  * \warning         Decryption with the piecewise API is discouraged, see the
  *                  warning on \c mbedtls_chachapoly_init().
  *
- * \param ctx       The ChaCha20-Poly1305 context.
+ * \param ctx       The ChaCha20-Poly1305 context. Must be initialized.
  * \param nonce     The nonce/IV to use for the message. Must be 12 bytes.
  * \param mode      The operation to perform: #MBEDTLS_CHACHAPOLY_ENCRYPT or
  *                  #MBEDTLS_CHACHAPOLY_DECRYPT (discouraged, see warning).
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA
- *                  if \p ctx or \p mac are NULL.
+ * \return          A negative error code on failure.
  */
 int mbedtls_chachapoly_starts( mbedtls_chachapoly_context *ctx,
                                const unsigned char nonce[12],
@@ -227,20 +228,19 @@
  * \warning         Decryption with the piecewise API is discouraged, see the
  *                  warning on \c mbedtls_chachapoly_init().
  *
- * \param ctx       The ChaCha20-Poly1305 context to use.
+ * \param ctx       The ChaCha20-Poly1305 context to use. Must be initialized.
  * \param len       The length (in bytes) of the data to encrypt or decrypt.
  * \param input     The buffer containing the data to encrypt or decrypt.
- *                  This pointer can be NULL if len == 0.
- * \param output    The buffer to where the encrypted or decrypted data is written.
- *                  Must be able to hold \p len bytes.
- *                  This pointer can be NULL if len == 0.
+ *                  This pointer can be \c NULL if `len == 0`.
+ * \param output    The buffer to where the encrypted or decrypted data is
+ *                  written. Must be able to hold \p len bytes.
+ *                  This pointer can be \c NULL if `len == 0`.
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA
- *                  if \p ctx, \p input, or \p output are NULL.
  * \return          #MBEDTLS_ERR_CHACHAPOLY_BAD_STATE
  *                  if the operation has not been started or has been
  *                  finished.
+ * \return          Another negative error code on other kinds of failure.
  */
 int mbedtls_chachapoly_update( mbedtls_chachapoly_context *ctx,
                                size_t len,
@@ -251,18 +251,17 @@
  * \brief           This function finished the ChaCha20-Poly1305 operation and
  *                  generates the MAC (authentication tag).
  *
- * \param ctx       The ChaCha20-Poly1305 context to use.
+ * \param ctx       The ChaCha20-Poly1305 context to use. Must be initialized.
  * \param mac       The buffer to where the 128-bit (16 bytes) MAC is written.
  *
  * \warning         Decryption with the piecewise API is discouraged, see the
  *                  warning on \c mbedtls_chachapoly_init().
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA
- *                  if \p ctx or \p mac are NULL.
  * \return          #MBEDTLS_ERR_CHACHAPOLY_BAD_STATE
  *                  if the operation has not been started or has been
  *                  finished.
+ * \return          Another negative error code on other kinds of failure.
  */
 int mbedtls_chachapoly_finish( mbedtls_chachapoly_context *ctx,
                                unsigned char mac[16] );
@@ -280,20 +279,20 @@
  *                  and key.
  *
  * \param ctx       The ChaCha20-Poly1305 context to use (holds the key).
+ *                  Must be initialized.
  * \param length    The length (in bytes) of the data to encrypt or decrypt.
  * \param nonce     The 96-bit (12 bytes) nonce/IV to use.
  * \param aad       The buffer containing the additional authenticated data (AAD).
- *                  This pointer can be NULL if aad_len == 0.
+ *                  This pointer can be \c NULL if `aad_len == 0`.
  * \param aad_len   The length (in bytes) of the AAD data to process.
  * \param input     The buffer containing the data to encrypt or decrypt.
- *                  This pointer can be NULL if ilen == 0.
+ *                  This pointer can be \c NULL if `ilen == 0`.
  * \param output    The buffer to where the encrypted or decrypted data is written.
- *                  This pointer can be NULL if ilen == 0.
+ *                  This pointer can be \c NULL if `ilen == 0`.
  * \param tag       The buffer to where the computed 128-bit (16 bytes) MAC is written.
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA
- *                  if one or more of the required parameters are NULL.
+ * \return          A negative error code on failure.
  */
 int mbedtls_chachapoly_encrypt_and_tag( mbedtls_chachapoly_context *ctx,
                                         size_t length,
@@ -324,10 +323,9 @@
  *                  This pointer can be NULL if ilen == 0.
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA
- *                  if one or more of the required parameters are NULL.
  * \return          #MBEDTLS_ERR_CHACHAPOLY_AUTH_FAILED
  *                  if the data was not authentic.
+ * \return          Another negative error code on other kinds of failure.
  */
 int mbedtls_chachapoly_auth_decrypt( mbedtls_chachapoly_context *ctx,
                                      size_t length,