diff --git a/argument.cpp b/argument.cpp
index 38d3978..35f33e3 100644
--- a/argument.cpp
+++ b/argument.cpp
@@ -29,7 +29,8 @@
 ArgumentParser::ArgumentParser(int argc, char** argv)
 {
     auto option = 0;
-    while (-1 != (option = getopt_long(argc, argv, optionstr, options, NULL)))
+    while (-1 !=
+           (option = getopt_long(argc, argv, optionstr, options, nullptr)))
     {
         if ((option == '?') || (option == 'h'))
         {