diff --git a/include/mbedtls/x509_csr.h b/include/mbedtls/x509_csr.h
index c46bc61..0082ada 100644
--- a/include/mbedtls/x509_csr.h
+++ b/include/mbedtls/x509_csr.h
@@ -99,6 +99,7 @@
  * \param csr      CSR context to fill
  * \param buf      buffer holding the CRL data
  * \param buflen   size of the buffer
+ *                 (including the terminating null byte for PEM data)
  *
  * \return         0 if successful, or a specific X509 or PEM error code
  */