diff --git a/src/engine/csv-engine-listener.cc b/src/engine/csv-engine-listener.cc
index 3d0e9d0..5c45817 100644
--- a/src/engine/csv-engine-listener.cc
+++ b/src/engine/csv-engine-listener.cc
@@ -10,6 +10,10 @@ csv::csv_type const& CsvEngineListener::output() const {
     return output_;
 }
 
+csv::csv_type& CsvEngineListener::output() {
+    return output_;
+}
+
 void CsvEngineListener::on_acknowledgement(User user, UserOrderId id) {
     output_.emplace_back(csv::csv_line_type{
         "A",
diff --git a/src/engine/csv-engine-listener.hh b/src/engine/csv-engine-listener.hh
index a33c172..a7174e5 100644
--- a/src/engine/csv-engine-listener.hh
+++ b/src/engine/csv-engine-listener.hh
@@ -14,6 +14,7 @@ struct CsvEngineListener : EngineListener {
     virtual ~CsvEngineListener();
 
     csv::csv_type const& output() const;
+    csv::csv_type& output();
 
     /// Called when a new trade or cancel order has been acknowledged.
     void on_acknowledgement(User user, UserOrderId id) override;