CMSError: CMS 321 about python-gsmmodem HOT 3 OPEN

DEBUG: response: ['+CMGS: 40', 'OK']
SMS sent.

DEBUG: write: AT+CMGD=3,0
DEBUG: response: ['OK']
DEBUG: notification: ['+CDSI: "SR",8']
DEBUG: SMS status report received
DEBUG: write: AT+CPMS="SR"
DEBUG: response: ['+CPMS: 0,0,3,23,3,23', 'OK']
DEBUG: write: AT+CMGR=8
DEBUG: response: ['+CMS ERROR: 321']
Exception in thread Thread-11:
Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 892, in run
    self._target(*self._args, **self._kwargs)
  File "/home/proto/.local/lib/python3.9/site-packages/gsmmodem/modem.py", line 1205, in __threadedHandleModemNotification
    self._handleSmsStatusReport(line)
  File "/home/proto/.local/lib/python3.9/site-packages/gsmmodem/modem.py", line 1373, in _handleSmsStatusReport
    report = self.readStoredSms(msgIndex, msgMemory)
  File "/home/proto/.local/lib/python3.9/site-packages/gsmmodem/modem.py", line 1428, in readStoredSms
    msgData = self.write('AT+CMGR={0}'.format(index))
  File "/home/proto/.local/lib/python3.9/site-packages/gsmmodem/modem.py", line 492, in write
    raise CmsError(data, int(errorCode))
gsmmodem.exceptions.CmsError: CMS 321