diff --git a/GridKit/Model/VariableMonitor.hpp b/GridKit/Model/VariableMonitor.hpp
index ce7df4f4..c50654ae 100644
--- a/GridKit/Model/VariableMonitor.hpp
+++ b/GridKit/Model/VariableMonitor.hpp
@@ -9,6 +9,7 @@
 #include <limits>
 #include <memory>
 #include <set>
+#include <sstream>
 #include <string>
 #include <type_traits>
 #include <variant>