state property is empty

I'm trying to add to qstatemachine a property with string and an enum, but the result from the machine.property() is empty.
QStatemachine m_machine ;
idle->assignProperty(&m_machine, "state", Z_IDLE);
cool->assignProperty(&m_machine, "state", Z_COOL);
start_z->assignProperty(&m_machine, "state", Z_START);

QVariant st = m_machine.property("state");
QString s = st.toString();