EOFError,how to solve?

Traceback (most recent call last):
File "calibre/utils/ipc/simple_worker.py", line 53, in run
File "calibre/utils/ipc/__init__.py", line 20, in eintr_retry_call
File "multiprocessing/connection.py", line 250, in recv
File "multiprocessing/connection.py", line 430, in _recv_bytes
File "multiprocessing/connection.py", line 399, in _recv
EOFError

here is my code:

futures = [self.thread_pool.submit(self.mymethod,id) for id in ids]

class MYCLASS:
def __init__(self, config, log=None):
...
self.lock = threading.Lock()

def mymethod(self,id):
...
with self.lock:
cover_pixmap = QPixmap()
cover_pixmap.load('/Volumes/data/bookcovers/' + id + '.jpg')
book['cover_pixmap'] = cover_pixmap
...