diff --git a/src/util/PlaceholderString.h b/src/util/PlaceholderString.h
index 9c0d226c..3021d235 100644
--- a/src/util/PlaceholderString.h
+++ b/src/util/PlaceholderString.h
@@ -17,6 +17,8 @@ using std::string;
 #include <vector>
 using std::vector;
 
+#include <ostream>
+
 class PlaceholderElement;
 
 /**