FTP error on Quit command

I'm using a ftp library that causes an error sending the quit command. It seems that the character before QUIT is not recognized. I took a look at the code but didn't find anything.

500 '?QUIT': command not understo开发者_如何学编程od

Does anyone know what this character can be?

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You can probably use a network sniffer to monitor the traffic over the FTP port (21) and check what is the actual character (typically in Hex format).