diff --git a/include/nana/gui/widgets/listbox.hpp b/include/nana/gui/widgets/listbox.hpp
index 212be2f7..e29cd1f4 100644
--- a/include/nana/gui/widgets/listbox.hpp
+++ b/include/nana/gui/widgets/listbox.hpp
@@ -535,7 +535,7 @@ namespace nana
 						throw std::invalid_argument("invalid listbox model container type");
 
 					if (nullptr == p->pointer())
-						throw std::runtime_error("the modal is immutable");
+						throw std::runtime_error("the modal is immutable, please declare model_guard with const");
 
 					return *static_cast<stlcontainer*>(p->pointer());
 				}