diff --git a/include/mbedtls/sha1.h b/include/mbedtls/sha1.h
index 801759a..0979dc0 100644
--- a/include/mbedtls/sha1.h
+++ b/include/mbedtls/sha1.h
@@ -141,7 +141,6 @@
  *                 and have a hash operation started.
  * \param input    The buffer holding the input data.
  *                 This must be a readable buffer of length \p ilen Bytes.
- *                 It may be \c NULL if \p ilen is zero.
  * \param ilen     The length of the input data \p input in Bytes.
  *
  * \return         \c 0 on success.
@@ -224,7 +223,6 @@
  *                 have a hash operation started.
  * \param input    The buffer holding the input data.
  *                 This must be a readable buffer of length \p ilen Bytes.
- *                 It may be \c NULL if \p ilen is zero.
  * \param ilen     The length of the input data \p input in Bytes.
  *
  */
@@ -286,7 +284,6 @@
  *
  * \param input    The buffer holding the input data.
  *                 This must be a readable buffer of length \p ilen Bytes.
- *                 It may be \c NULL if \p ilen is zero.
  * \param ilen     The length of the input data \p input in Bytes.
  * \param output   The SHA-1 checksum result.
  *                 This must be a writable buffer of length \c 20 Bytes.
@@ -322,7 +319,6 @@
  *
  * \param input    The buffer holding the input data.
  *                 This must be a readable buffer of length \p ilen Bytes.
- *                 It may be \c NULL if \p ilen is zero.
  * \param ilen     The length of the input data \p input in Bytes.
  * \param output   The SHA-1 checksum result. This must be a writable
  *                 buffer of size \c 20 Bytes.