diff --git a/jerry-ext/arg/arg-transform-functions.c b/jerry-ext/arg/arg-transform-functions.c
index d9ecb52b0..1d609660d 100644
--- a/jerry-ext/arg/arg-transform-functions.c
+++ b/jerry-ext/arg/arg-transform-functions.c
@@ -132,7 +132,7 @@ jerryx_arg_helper_process_double (double *d, /**< [in, out] the number to be pro
                                   double max, /**< the max value for clamping */
                                   jerryx_arg_int_option_t option) /**< the converting policies */
 {
-  if (isnan (*d))
+  if (*d != *d) /* isnan (*d) triggers conversion warning on clang<9 */
   {
     return jerry_create_error (JERRY_ERROR_TYPE,
                                (jerry_char_t *) "The number is NaN.");