diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index 336f2da..8ecab4e 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -100,6 +100,7 @@
  * \param p         The reference to the current position pointer.
  * \param start     The start of the buffer, for bounds-checking.
  * \param X         The MPI to write.
+ *                  It must be non-negative.
  *
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
@@ -184,6 +185,7 @@
  * \param p         The reference to the current position pointer.
  * \param start     The start of the buffer, for bounds-checking.
  * \param val       The integer value to write.
+ *                  It must be non-negative.
  *
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
@@ -232,7 +234,7 @@
 
 /**
  * \brief           Write a UTF8 string in ASN.1 format using the UTF8String
- *                  string encoding tag (#MBEDTLS_ASN1_PRINTABLE_STRING).
+ *                  string encoding tag (#MBEDTLS_ASN1_UTF8_STRING).
  *
  * \note            This function works backwards in data buffer.
  *