diff --git a/kernel/register.h b/kernel/register.h
index 9b247172..71ab6ea6 100644
--- a/kernel/register.h
+++ b/kernel/register.h
@@ -101,7 +101,7 @@ struct Backend : Pass
 // implemented in passes/cmds/select.cc
 extern void handle_extra_select_args(Pass *pass, std::vector<std::string> args, size_t argidx, size_t args_size, RTLIL::Design *design);
 extern RTLIL::Selection eval_select_args(const vector<string> &args, RTLIL::Design *design);
-extern void eval_select_op(vector<RTLIL::Selection> &work, string &op, RTLIL::Design *design);
+extern void eval_select_op(vector<RTLIL::Selection> &work, const string &op, RTLIL::Design *design);
 
 extern std::map<std::string, Pass*> pass_register;
 extern std::map<std::string, Frontend*> frontend_register;
diff --git a/passes/cmds/select.cc b/passes/cmds/select.cc
index f8cfa9ce..247765f0 100644
--- a/passes/cmds/select.cc
+++ b/passes/cmds/select.cc
@@ -839,7 +839,7 @@ RTLIL::Selection eval_select_args(const vector<string> &args, RTLIL::Design *des
 }
 
 // extern decl. in register.h
-void eval_select_op(vector<RTLIL::Selection> &work, string &op, RTLIL::Design *design)
+void eval_select_op(vector<RTLIL::Selection> &work, const string &op, RTLIL::Design *design)
 {
 	work_stack.swap(work);
 	select_stmt(design, op);