diff --git a/httplib.h b/httplib.h
index d3817a6..deb17b2 100644
--- a/httplib.h
+++ b/httplib.h
@@ -196,6 +196,7 @@ using socket_t = int;
 #include <string>
 #include <sys/stat.h>
 #include <thread>
+#include <cctype>
 
 #ifdef CPPHTTPLIB_OPENSSL_SUPPORT
 #include <openssl/err.h>