diff --git a/xo-interpreter2/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp b/xo-interpreter2/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp
index 9f8f49d8..46de95f3 100644
--- a/xo-interpreter2/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp
+++ b/xo-interpreter2/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp
@@ -119,7 +119,7 @@ namespace xo {
             /** evaluate expression @p expr
              *  Require: must first start interactive/batch session
              **/
-            VsmResult start_eval(obj<AExpression> expr);
+            const VsmResult & start_eval(obj<AExpression> expr);
 
             /** borrow calling thread to run indefinitely,
              *  until halt instruction
diff --git a/xo-interpreter2/src/interpreter2/DVirtualSchematikaMachine.cpp b/xo-interpreter2/src/interpreter2/DVirtualSchematikaMachine.cpp
index 5c3d35d2..8edc0538 100644
--- a/xo-interpreter2/src/interpreter2/DVirtualSchematikaMachine.cpp
+++ b/xo-interpreter2/src/interpreter2/DVirtualSchematikaMachine.cpp
@@ -225,7 +225,7 @@ namespace xo {
             return VsmResultExt(VsmResult(*p_value), remaining);
         }
 
-        VsmResult
+        const VsmResult &
         DVirtualSchematikaMachine::start_eval(obj<AExpression> expr)
         {
             this->pc_ = VsmInstr::c_eval;