diff --git a/src/App/Expression.cpp b/src/App/Expression.cpp
index bb3568e002..9c1bc60986 100644
--- a/src/App/Expression.cpp
+++ b/src/App/Expression.cpp
@@ -2467,7 +2467,7 @@ void FunctionExpression::_visit(ExpressionVisitor &v)
 
 TYPESYSTEM_SOURCE(App::VariableExpression, App::UnitExpression)
 
-VariableExpression::VariableExpression(const DocumentObject *_owner, ObjectIdentifier _var)
+VariableExpression::VariableExpression(const DocumentObject *_owner, const ObjectIdentifier& _var)
     : UnitExpression(_owner)
     , var(_var)
 {
diff --git a/src/App/ExpressionParser.h b/src/App/ExpressionParser.h
index f5b9545708..cbfb369034 100644
--- a/src/App/ExpressionParser.h
+++ b/src/App/ExpressionParser.h
@@ -321,7 +321,7 @@ protected:
 class AppExport VariableExpression : public UnitExpression {
     TYPESYSTEM_HEADER_WITH_OVERRIDE();
 public:
-    VariableExpression(const App::DocumentObject *_owner = 0, ObjectIdentifier _var = ObjectIdentifier());
+    VariableExpression(const App::DocumentObject *_owner = 0, const ObjectIdentifier& _var = ObjectIdentifier());
 
     ~VariableExpression();