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.