Add missing #include <iostream>
diff --git a/src/mutator.cc b/src/mutator.cc
index 77ce401..b29ea8f 100644
--- a/src/mutator.cc
+++ b/src/mutator.cc
@@ -16,6 +16,7 @@
 
 #include <algorithm>
 #include <bitset>
+#include <iostream>
 #include <map>
 #include <memory>
 #include <random>