reproduce OpenSSL error

from OpenSSL import crypto

# Generate key-pair
key = crypto.PKey()
key.generate_key(crypto.TYPE_RSA, 512)

# Generate CSR
req = crypto.X509Req()
req.get_subject().CN = "walrus.example.com"
req.set_pubkey(key)

# dump unsigned CSR
unsigned_csr_dump = crypto.dump_certificate_request(crypto.FILETYPE_PEM, req)
with open('unsigned.csr', 'w') as unsigned_csr:
    unsigned_csr.write(unsigned_csr_dump)

# sign the CSR
req.sign(key, "sha256")

# dump signed CSR
signed_csr_dump = crypto.dump_certificate_request(crypto.FILETYPE_PEM, req)
with open('signed.csr', 'w') as signed_csr:
    signed_csr.write(signed_csr_dump)

# If the CSR has the signature, it will successfully load.
csr = crypto.load_certificate_request(crypto.FILETYPE_PEM, signed_csr_dump)
print 'Loaded signed CSR'

# If the CSR doesn't have the signature, there will be an error while trying to load it
csr = crypto.load_certificate_request(crypto.FILETYPE_PEM, unsigned_csr_dump)
# The code won't get here
print 'Loaded unsigned CSR'