diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 810873c..f88e8cc 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -1013,17 +1013,17 @@
  * \brief           Set the serial number for a Certificate.
  *
  * \param ctx               CRT context to use
- * \param serial_buff       Input buffer containing the serial number in big
- *                          endian format
+ * \param serial_buff       A raw array of bytes containing the serial number
+ *                          in big endian format
  * \param serial_buff_len   Length of the previous input buffer buffer
  *
  * \return          0 if successful, or
- *                  MBEDTLS_ERR_X509_BAD_INPUT_DATA if the provided input buffer:
- *                  - is too big (longer than MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN)
- *                  - contains invalid chars
+ *                  MBEDTLS_ERR_X509_BAD_INPUT_DATA if the provided input buffer
+ *                  is too big (longer than MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN)
  */
 int mbedtls_x509write_crt_set_serial_new(mbedtls_x509write_cert *ctx,
-                                         char *serial_buff, size_t serial_buff_len);
+                                         unsigned char *serial_buff,
+                                         size_t serial_buff_len);
 
 /**
  * \brief           Set the validity period for a Certificate