Closing an RS232 serial port connection, if open

Hello. I am using the following code to open, send a command and receive a response - it works fine:

s = serial('COM1');
set(s,'BaudRate',4800);
fopen(s);
fprintf(s,'o')
out = fscanf(s);

What is the best way to close, delete or remove the serial object to prevent my external device hanging up:

I have an exit button where I would need to pass the s serial object and then do what ever is the appropriate operation. However, not all of the times I open the coms port, so I would need to account for cases where the coms port is not open.

Thanks for any advice. Jason

[Jason]

Just to add I have used:

fclose(s)
delete(s)

But this causes matlab to hang.