diff --git a/src/std_include.hpp b/src/std_include.hpp
index 35ec577..9e83e23 100644
--- a/src/std_include.hpp
+++ b/src/std_include.hpp
@@ -57,6 +57,7 @@
 
 #include <atomic>
 #include <cassert>
+#include <cctype>
 #include <chrono>
 #include <csignal>
 #include <cstdarg>