diff --git a/library/x509_crt.c b/library/x509_crt.c
index 57e772d..ee9e95d 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -958,7 +958,7 @@
             ret = MBEDTLS_ERR_ASN1_UNEXPECTED_TAG;
             goto err;
         }
-#endif
+#endif /* MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION */
         return( 0 );
     }