# Buggy code
def sanity_check(module, host, key, sshkeygen):
    """Check supplied key is sensible

    host and key are parameters provided by the user; If the host
    provided is inconsistent with the key supplied, then this function
    quits, providing an error to the user.
    sshkeygen is the path to ssh-keygen, found earlier with get_bin_path
    """
    # If no key supplied, we're doing a removal, and have nothing to check here.
    if key is None:
        return
    # Rather than parsing the key ourselves, get ssh-keygen to do it
    # (this is essential for hashed keys, but otherwise useful, as the
    # key question is whether ssh-keygen thinks the key matches the host).

    # The approach is to write the key to a temporary file,
    # and then attempt to look up the specified host in that file.

    if re.search(r"\S+(\s+)?,(\s+)?", host):
        module.fail_json(
            msg="Comma separated list of names is not supported. "
            "Please pass a single name to lookup in the known_hosts file."
        )

    try:
        outf = tempfile.NamedTemporaryFile(mode="w+")
        outf.write(key)
        outf.flush()
    except IOError as e:
        module.fail_json(
            msg="Failed to write to temporary file %s: %s" % (outf.name, to_native(e))
        )

    sshkeygen_command = [sshkeygen, "-F", host, "-f", outf.name]
    rc, stdout, stderr = module.run_command(sshkeygen_command)
    try:
        outf.close()
    except:
        pass

    if stdout == "":  # host not found
        module.fail_json(
            msg="Host parameter does not match hashed host field in supplied key"
        )


# Traceback (IndexError)
The full traceback is:
Traceback (most recent call last):
File "/tmp/ansible_nz17ajoh/ansible_module_known_hosts.py", line 332, in <module>
main()
File "/tmp/ansible_nz17ajoh/ansible_module_known_hosts.py", line 327, in main
results = enforce_state(module, module.params)
File "/tmp/ansible_nz17ajoh/ansible_module_known_hosts.py", line 106, in enforce_state
found, replace_or_add, found_line, key = search_for_host_key(module, host, key, hash_host, path, sshkeygen)
File "/tmp/ansible_nz17ajoh/ansible_module_known_hosts.py", line 254, in search_for_host_key
hashed_host = normalize_known_hosts_key(hashed_lines[lnum])
File "/tmp/ansible_nz17ajoh/ansible_module_known_hosts.py", line 278, in normalize_known_hosts_key
if k[0][0] == '@':
IndexError: list index out of range

# Fixed code
def sanity_check(module, host, key, sshkeygen):
    """Check supplied key is sensible

    host and key are parameters provided by the user; If the host
    provided is inconsistent with the key supplied, then this function
    quits, providing an error to the user.
    sshkeygen is the path to ssh-keygen, found earlier with get_bin_path
    """
    # If no key supplied, we're doing a removal, and have nothing to check here.
    if not key:
        return
    # Rather than parsing the key ourselves, get ssh-keygen to do it
    # (this is essential for hashed keys, but otherwise useful, as the
    # key question is whether ssh-keygen thinks the key matches the host).

    # The approach is to write the key to a temporary file,
    # and then attempt to look up the specified host in that file.

    if re.search(r"\S+(\s+)?,(\s+)?", host):
        module.fail_json(
            msg="Comma separated list of names is not supported. "
            "Please pass a single name to lookup in the known_hosts file."
        )

    with tempfile.NamedTemporaryFile(mode="w+") as outf:
        try:
            outf.write(key)
            outf.flush()
        except IOError as e:
            module.fail_json(
                msg="Failed to write to temporary file %s: %s"
                % (outf.name, to_native(e))
            )

        sshkeygen_command = [sshkeygen, "-F", host, "-f", outf.name]
        rc, stdout, stderr = module.run_command(sshkeygen_command)

    if stdout == "":  # host not found
        module.fail_json(
            msg="Host parameter does not match hashed host field in supplied key"
        )