diff --git a/lstm/lstmtrainer.cpp b/lstm/lstmtrainer.cpp
index 42e43162ce..3976b8a79c 100644
--- a/lstm/lstmtrainer.cpp
+++ b/lstm/lstmtrainer.cpp
@@ -147,7 +147,7 @@ void LSTMTrainer::InitCharSet(const UNICHARSET& unicharset,
 // ctc_mode controls how the truth text is mapped to the network targets.
 // Note: Call before InitNetwork!
 void LSTMTrainer::InitCharSet(const UNICHARSET& unicharset,
-                              const UnicharCompress recoder) {
+                              const UnicharCompress& recoder) {
   EmptyConstructor();
   int flags = TF_COMPRESS_UNICHARSET;
   training_flags_ = static_cast<TrainingFlags>(flags);
diff --git a/lstm/lstmtrainer.h b/lstm/lstmtrainer.h
index 918c0381be..c60405076d 100644
--- a/lstm/lstmtrainer.h
+++ b/lstm/lstmtrainer.h
@@ -113,7 +113,7 @@ class LSTMTrainer : public LSTMRecognizer {
   // previously setup UNICHARSET and UnicharCompress.
   // ctc_mode controls how the truth text is mapped to the network targets.
   // Note: Call before InitNetwork!
-  void InitCharSet(const UNICHARSET& unicharset, const UnicharCompress recoder);
+  void InitCharSet(const UNICHARSET& unicharset, const UnicharCompress& recoder);
 
   // Initializes the trainer with a network_spec in the network description
   // net_flags control network behavior according to the NetworkFlags enum.