diff --git a/include/polarssl/x509.h b/include/polarssl/x509.h
index adaacff..de487ce 100644
--- a/include/polarssl/x509.h
+++ b/include/polarssl/x509.h
@@ -76,6 +76,7 @@
 #define POLARSSL_ERR_X509_BAD_INPUT_DATA                   -0x2800  /**< Input invalid. */
 #define POLARSSL_ERR_X509_MALLOC_FAILED                    -0x2880  /**< Allocation of memory failed. */
 #define POLARSSL_ERR_X509_FILE_IO_ERROR                    -0x2900  /**< Read/write of file failed. */
+#define POLARSSL_ERR_X509_FATAL_ERROR                      -0x3000  /**< A fatal error occured, eg the chain is too long or the vrfy callback failed. */
 /* \} name */
 
 /**
diff --git a/include/polarssl/x509_crt.h b/include/polarssl/x509_crt.h
index 24f7c7a..eef7f3a 100644
--- a/include/polarssl/x509_crt.h
+++ b/include/polarssl/x509_crt.h
@@ -232,7 +232,13 @@
  *
  *                 All flags left after returning from the callback
  *                 are also returned to the application. The function should
- *                 return 0 for anything but a fatal error.
+ *                 return 0 for anything (including invalid certificates)
+ *                 other than fatal error, as a non-zero return code
+ *                 immediately aborts the verification process. For fatal
+ *                 errors, a specific error code should be used (different
+ *                 from POLARSSL_ERR_X509_CERT_VERIFY_FAILED which should not
+ *                 be returned at this point), or POLARSSL_ERR_X509_FATAL_ERROR
+ *                 can be used if no better code is available.
  *
  * \note           In case verification failed, the results can be displayed
  *                 using \c x509_crt_verify_info()