diff --git a/src/math/simplex/model_based_opt.cpp b/src/math/simplex/model_based_opt.cpp
index 8a771e07c..4053c41b7 100644
--- a/src/math/simplex/model_based_opt.cpp
+++ b/src/math/simplex/model_based_opt.cpp
@@ -606,7 +606,7 @@ namespace opt {
         }
     }
 
-    void model_based_opt::mk_coeffs_without(vector<var>& dst, vector<var> const src, unsigned x) {
+    void model_based_opt::mk_coeffs_without(vector<var>& dst, vector<var> const& src, unsigned x) {
         for (var const & v : src) {
             if (v.m_id != x) dst.push_back(v);
         }
diff --git a/src/math/simplex/model_based_opt.h b/src/math/simplex/model_based_opt.h
index e52d0cfe0..30442fc58 100644
--- a/src/math/simplex/model_based_opt.h
+++ b/src/math/simplex/model_based_opt.h
@@ -132,7 +132,7 @@ namespace opt {
 
         void normalize(unsigned row_id);
 
-        void mk_coeffs_without(vector<var>& dst, vector<var> const src, unsigned x);
+        void mk_coeffs_without(vector<var>& dst, vector<var> const& src, unsigned x);
 
         unsigned new_row();