foolscap Violation: None is not a bytestring

[exarkun]

On a ZKAPAuthorizer-enabled Tahoe-LAFS client node sometimes this is logged:

2022-02-01T10:58:12-0500 [-] Fuzzy timer service (lease maintenance service)
        Traceback (most recent call last):
          File "/nix/store/agy6yqqybdwh6scvmavkrsr1kx4dfrmm-python3-3.9.6-env/lib/python3.9/site-packages/twisted/internet/defer.py", line 501, in errback
            self._startRunCallbacks(fail)
          File "/nix/store/agy6yqqybdwh6scvmavkrsr1kx4dfrmm-python3-3.9.6-env/lib/python3.9/site-packages/twisted/internet/defer.py", line 568, in _startRunCallbacks
            self._runCallbacks()
          File "/nix/store/agy6yqqybdwh6scvmavkrsr1kx4dfrmm-python3-3.9.6-env/lib/python3.9/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/nix/store/agy6yqqybdwh6scvmavkrsr1kx4dfrmm-python3-3.9.6-env/lib/python3.9/site-packages/twisted/internet/defer.py", line 1475, in gotResult
            _inlineCallbacks(r, g, status)
        --- <exception caught here> ---
          File "/nix/store/agy6yqqybdwh6scvmavkrsr1kx4dfrmm-python3-3.9.6-env/lib/python3.9/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
            result = g.send(result)
          File "/nix/store/agy6yqqybdwh6scvmavkrsr1kx4dfrmm-python3-3.9.6-env/lib/python3.9/site-packages/_zkapauthorizer/controller.py", line 1030, in bracket
            result = yield between()
        foolscap.tokens.Violation: Violation (RIPrivacyPassAuthorizedStorageServer.tahoe.privatestorage.io.stat_shares(storage_indexes_or_slots=)): ("'None' is not a bytestring",)