Exception when sending mine request

I get following Exception in my terminal when executing the mine request:

`Exception happened during processing of request from ('127.0.0.1', 57087)
Traceback (most recent call last):
File "/Users/chris/anaconda/lib/python3.5/socketserver.py", line 313, in _handle_request_noblock
self.process_request(request, client_address)
File "/Users/chris/anaconda/lib/python3.5/socketserver.py", line 341, in process_request
self.finish_request(request, client_address)
File "/Users/chris/anaconda/lib/python3.5/socketserver.py", line 354, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/Users/chris/anaconda/lib/python3.5/socketserver.py", line 681, in __init__
self.handle()
File "/Users/chris/anaconda/lib/python3.5/site-packages/werkzeug/serving.py", line 293, in handle
rv = BaseHTTPRequestHandler.handle(self)
File "/Users/chris/anaconda/lib/python3.5/http/server.py", line 422, in handle
self.handle_one_request()
File "/Users/chris/anaconda/lib/python3.5/site-packages/werkzeug/serving.py", line 328, in handle_one_request
return self.run_wsgi()
File "/Users/chris/anaconda/lib/python3.5/site-packages/werkzeug/serving.py", line 270, in run_wsgi
execute(self.server.app)
File "/Users/chris/anaconda/lib/python3.5/site-packages/werkzeug/serving.py", line 258, in execute
application_iter = app(environ, start_response)
File "/Users/chris/anaconda/lib/python3.5/site-packages/flask/app.py", line 1997, in __call__
return self.wsgi_app(environ, start_response)
File "/Users/chris/anaconda/lib/python3.5/site-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/Users/chris/anaconda/lib/python3.5/site-packages/flask/app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "/Users/chris/anaconda/lib/python3.5/site-packages/flask/app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "blockchain.py", line 208, in mine
proof = blockchain.proof_of_work(last_block)
File "blockchain.py", line 172, in proof_of_work
while self.valid_proof(last_proof, proof, last_hash) is False:
File "blockchain.py", line 189, in valid_proof
guess = '{last_proof}{proof}{last_hash}'.encode()`