diff --git a/include/ignition/math/graph/Graph.hh b/include/ignition/math/graph/Graph.hh
index cae3170e1..3c7ca10fc 100644
--- a/include/ignition/math/graph/Graph.hh
+++ b/include/ignition/math/graph/Graph.hh
@@ -18,6 +18,7 @@
 #define IGNITION_MATH_GRAPH_GRAPH_HH_
 
 #include <cassert>
+#include <iostream>
 #include <map>
 #include <ostream>
 #include <set>