diff --git a/src/solvers/refinement/string_constraint_generator_testing.cpp b/src/solvers/refinement/string_constraint_generator_testing.cpp
index 829c1d21887..bb7f7418330 100644
--- a/src/solvers/refinement/string_constraint_generator_testing.cpp
+++ b/src/solvers/refinement/string_constraint_generator_testing.cpp
@@ -95,8 +95,8 @@ exprt string_constraint_generatort::add_axioms_for_is_prefix(
   const function_application_exprt::argumentst &args=f.arguments();
   PRECONDITION(f.type()==bool_typet() || f.type().id()==ID_c_bool);
   PRECONDITION(args.size() == 2 || args.size() == 3);
-  const array_string_exprt s0 = get_string_expr(args[swap_arguments ? 1 : 0]);
-  const array_string_exprt s1 = get_string_expr(args[swap_arguments ? 0 : 1]);
+  const array_string_exprt &s0 = get_string_expr(args[swap_arguments ? 1 : 0]);
+  const array_string_exprt &s1 = get_string_expr(args[swap_arguments ? 0 : 1]);
   const exprt offset =
     args.size() == 2 ? from_integer(0, s0.length().type()) : args[2];
   return typecast_exprt(add_axioms_for_is_prefix(s0, s1, offset), f.type());