diff --git a/src/Episode3/RulerServer.cc b/src/Episode3/RulerServer.cc
index bbbdfea8..eb650a35 100644
--- a/src/Episode3/RulerServer.cc
+++ b/src/Episode3/RulerServer.cc
@@ -1,5 +1,7 @@
 #include "RulerServer.hh"
 
+#include <optional>
+
 #include "Server.hh"
 
 using namespace std;