diff --git a/json_tokener.c b/json_tokener.c
index 416058d..620468e 100644
--- a/json_tokener.c
+++ b/json_tokener.c
@@ -403,7 +403,7 @@ struct json_object* json_tokener_parse_ex(struct json_tokener *tok,
 	{
 		char inf_char = *str;
 		if (!(tok->flags & JSON_TOKENER_STRICT))
-			inf_char = tolower(*str);
+			inf_char = tolower((int)*str);
 		if (inf_char != _json_inf_str[tok->st_pos])
 		{
 			tok->err = json_tokener_error_parse_unexpected;