diff --git a/src/app/gui/zeml.h b/src/app/gui/zeml.h
index ef8ba610..89e8c974 100644
--- a/src/app/gui/zeml.h
+++ b/src/app/gui/zeml.h
@@ -4,6 +4,7 @@
 #include <imgui/imgui.h>
 
 #include <cctype>
+#include <functional>
 #include <map>
 #include <sstream>
 #include <string>