diff --git a/src/backend/concrete_history.cc b/src/backend/concrete_history.cc
index 1b491a1..004549c 100644
--- a/src/backend/concrete_history.cc
+++ b/src/backend/concrete_history.cc
@@ -44,6 +44,10 @@
 #include <cassert>
 #include <functional>
 
+#ifndef NDEBUG
+#include <iostream>
+#endif
+
 using namespace sgpem;
 using namespace std;
 using memory::smart_ptr;