diff --git a/library/bignum.c b/library/bignum.c
index 76202fe51..521787d74 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1933,10 +1933,9 @@ cleanup:
 
 /*
  * Fill X with size bytes of random.
- *
- * Use a temporary bytes representation to make sure the result is the same
- * regardless of the platform endianness (useful when f_rng is actually
- * deterministic, eg for tests).
+ * The bytes returned from the RNG are used in a specific order which
+ * is suitable for deterministic ECDSA (see the specification of
+ * mbedtls_mpi_random() and the implementation in mbedtls_mpi_fill_random()).
  */
 int mbedtls_mpi_fill_random( mbedtls_mpi *X, size_t size,
                      int (*f_rng)(void *, unsigned char *, size_t),
diff --git a/library/bignum_core.c b/library/bignum_core.c
index e40599596..b990a8dcc 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -557,9 +557,8 @@ void mbedtls_mpi_core_ct_uint_table_lookup( mbedtls_mpi_uint *dest,
 /* Fill X with n_bytes random bytes.
  * X must already have room for those bytes.
  * The ordering of the bytes returned from the RNG is suitable for
- * deterministic ECDSA (see RFC 6979 §3.3 and mbedtls_mpi_core_random()).
- * The size and sign of X are unchanged.
- * n_bytes must not be 0.
+ * deterministic ECDSA (see RFC 6979 §3.3 and the specification of
+ * mbedtls_mpi_core_random()).
  */
 int mbedtls_mpi_core_fill_random(
     mbedtls_mpi_uint *X, size_t X_limbs,