You are correct that there is no tax (assuming you have no other taxable assets there). But you need to pay the fee and file the return. It will take almost zero effort. Maryland will not allow businesses to INITIATE a court action in the state unless they maintain good standing. This is true whether you are a domestic MD company or a foreign (out of state) company, registered in MD as foreign. I have owned residential real estate property as both a domestic and foreign LLC in Maryland, and I file my return every year with zero tax due, and pay the fee to keep in good standing. If you don't file the real personal property return, your company will lose its good standing certification the next year.
