Upgrade to 13.0 fails

[ChrisRM]

Trying to do an upgrade from 12.0-U8.1 to 13.0, I keep hitting this on the initial load. I've tried re-downloading 3 or 4 times, the hashes all match, but it just doesn't want to start running. What have I got wrong?

Error: Traceback (most recent call last):
File "/usr/local/lib/freenasOS/Update.py", line 1258, in ExtractFrozenUpdate
with tarfile.open(tarball) as tf:
File "/usr/local/lib/python3.9/tarfile.py", line 1616, in open
raise ReadError("file could not be opened successfully")
tarfile.ReadError: file could not be opened successfully

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/middlewared/job.py", line 367, in run
await self.future
File "/usr/local/lib/python3.9/site-packages/middlewared/job.py", line 403, in __run_body
rv = await self.method(*([self] + args))
File "/usr/local/lib/python3.9/site-packages/middlewared/schema.py", line 975, in nf
return await f(*args, **kwargs)
File "/usr/local/lib/python3.9/site-packages/middlewared/plugins/update.py", line 389, in file
await self.middleware.call('update.install_manual_impl', job, destfile, dest_extracted)
File "/usr/local/lib/python3.9/site-packages/middlewared/main.py", line 1256, in call
return await self._call(
File "/usr/local/lib/python3.9/site-packages/middlewared/main.py", line 1224, in _call
return await self.run_in_executor(prepared_call.executor, methodobj, *prepared_call.args)
File "/usr/local/lib/python3.9/site-packages/middlewared/main.py", line 1128, in run_in_executor
return await loop.run_in_executor(pool, functools.partial(method, *args, **kwargs))
File "/usr/local/lib/python3.9/concurrent/futures/thread.py", line 52, in run
result = self.fn(*self.args, **self.kwargs)
File "/usr/local/lib/python3.9/site-packages/middlewared/plugins/update_/install_freebsd.py", line 56, in install_manual_impl
Update.ExtractFrozenUpdate(path, dest_extracted, verbose=True)
File "/usr/local/lib/freenasOS/Update.py", line 1278, in ExtractFrozenUpdate
raise Exceptions.UpdateBadFrozenFile("Bad tar file {0}".format(tarball))
NameError: name 'Exceptions' is not defined