diff --git a/src/Calculator/Calculator.h b/src/Calculator/Calculator.h
index 3d8b925..a433431 100644
--- a/src/Calculator/Calculator.h
+++ b/src/Calculator/Calculator.h
@@ -6,6 +6,7 @@
 #define CALCULATOR_H
 
 #include <map>
+#include <string>
 
 namespace Calculator {