Call history:

        mailbox-threads.scm:46: thread?
        mailbox-threads.scm:37: thread?
        mailbox-threads.scm:38: thread-specific
        mailbox-threads.scm:40: thread-specific
        <eval>    (mutex-unlock!121 mtx117)
        <eval>    (tcp-abandon-port c)
        <eval>    (dynamic-wind118 (lambda119 () (mutex-lock!120 mtx117)) (lambda119 () (delete (current-thread) mailb......
        <eval>    (mutex-lock!120 mtx117)
        <eval>    (delete (current-thread) mailboxes)
        <eval>    (current-thread)
        mailbox-threads.scm:46: thread?
        mailbox-threads.scm:37: thread?
        mailbox-threads.scm:38: thread-specific
        mailbox-threads.scm:40: thread-specific
        <eval>    (mutex-unlock!121 mtx117)
        <eval>    (tcp-abandon-port c)  <--