diff --git a/exercises/classes/classes.cpp b/exercises/classes/classes.cpp
index 15e24023..e3cba9fe 100644
--- a/exercises/classes/classes.cpp
+++ b/exercises/classes/classes.cpp
@@ -1,5 +1,6 @@
 #include <iomanip>
 #include <iostream>
+#include <sstream>
 #include <numeric>
 
 class Fraction {