ASP.Net C# and Active Directory error "c# index was out of range. Must be nonnegative and less than the size of the collection"

hello Experts,

I have a bit of code that binds to on Active Directory user, disable it, and move it to  a cleanup OU. I tried to add a line of code that will write to the description attriubute on the object, but I keep getting the error "c# index was out of range. Must be nonnegative and less than the size of the collection", once the code reaches userMove.Properties["description"][0] .
Below is a snipped of the code"

                                try
                                {
                                    AccDisable ad = new AccDisable();
                                    ad.Disable(userDN);
                                // Bind to the object to be moved.
                                    DirectoryEntry userMove = new DirectoryEntry("LDAP://" + userDN, systemUser, systemPw);
                                    // Update the Description attribute on the user
                                    userMove.Properties["description"][0] = "no NASW Authentication has taken place in the past" + dayCreatRound;
                                  

                                // Use the MoveTo property to define the new container you want to move the object to.
                                   // userMove.MoveTo(new DirectoryEntry("LDAP://OU=OVER_45_DAYS_USER,OU=CLEANUP,OU=domain.com", systemUser, systemPw));                           
                                }
                                catch (InvalidOperationException InvOpEx)
                                {
                                    // Handle InvalidOperationException.
                            
                                    string messageEx = "Could not move "+InvOpEx.Message;
                                    lm.WriteLogNoDS(messageEx,logLocation);
                                }
                                catch ( Exception Ex)
                                {
                                    // Handle all other errors
                                    string messageEx = "Could not move " + Ex.Message;
                                    lm.WriteLogNoDS(messageEx, logLocation);
                                }
                                #endregion
                            }

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