Unable to successfully sign ethereum transaction offline using TREZORCTL ethereum_sign_tx

I am trying to complete an offline ethereum transaction with a Trezor hardware wallet using https://github.com/trezor/python-trezor but I get the following error:

Traceback (most recent call last):
File "/usr/local/bin/trezorctl", line 851, in
cli()
File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 722, in call
return self.main(*args, **kwargs)
File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/usr/local/lib/python3.5/dist-packages/click/decorators.py", line 27, in new_func
return f(get_current_context().obj, *args, **kwargs)
File "/usr/local/bin/trezorctl", line 687, in ethereum_sign_tx
from ethjsonrpc import EthJsonRpc
File "/usr/local/lib/python3.5/dist-packages/ethjsonrpc/init.py", line 1, in
from ethjsonrpc.client import (EthJsonRpc, ParityEthJsonRpc,
File "/usr/local/lib/python3.5/dist-packages/ethjsonrpc/client.py", line 7, in
from ethereum import utils
File "/usr/local/lib/python3.5/dist-packages/ethereum/utils.py", line 103, in
assert sha3('').encode('hex') == 'c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470'
AttributeError: 'bytes' object has no attribute 'encode'

I've tried on different OS / VMs but had no luck. Not sure if this is a user error, or how to work around it. Any insight would be appreciated. I am sure I am using the correct syntax but there seems to be an issue with some of the dependencies, specifically when trying to successfully run **trezorctl ethereum_sign_tx** .

Thanks!