diff --git a/src/ccstruct/matrix.h b/src/ccstruct/matrix.h
index 6feec22336..7dc1bb153b 100644
--- a/src/ccstruct/matrix.h
+++ b/src/ccstruct/matrix.h
@@ -365,7 +365,7 @@ class GENERIC_2D_ARRAY {
   }
 
   // Accumulates the element-wise sums of squares of src into *this.
-  void SumSquares(const GENERIC_2D_ARRAY<T>& src, T decay_factor) {
+  void SumSquares(const GENERIC_2D_ARRAY<T>& src, const T& decay_factor) {
     T update_factor = 1.0 - decay_factor;
     int size = num_elements();
     for (int i = 0; i < size; ++i) {
@@ -377,7 +377,7 @@ class GENERIC_2D_ARRAY {
   // Scales each element using the adam algorithm, ie array_[i] by
   // sqrt(sqsum[i] + epsilon)).
   void AdamUpdate(const GENERIC_2D_ARRAY<T>& sum,
-                  const GENERIC_2D_ARRAY<T>& sqsum, T epsilon) {
+                  const GENERIC_2D_ARRAY<T>& sqsum, const T& epsilon) {
     int size = num_elements();
     for (int i = 0; i < size; ++i) {
       array_[i] += sum.array_[i] / (sqrt(sqsum.array_[i]) + epsilon);