Identify the mistake in this Python webhook signature verification code:

Identify the mistake in this Python webhook signature verification code:

import hmac
import hashlib

secret = b'secretkey'
payload = b'webhook payload'
header_signature = 'abcdef123456'

computed_signature = hmac.new(secret, payload, hashlib.sha256).hexdigest

if hmac.compare_digest(computed_signature, header_signature):
    print('Signature valid')
else:
    print('Signature invalid')