# Buggy code
def enforce_state(module, params):
    """
    Add or remove key.
    """

    host = params["name"].lower()
    key = params.get("key", None)
    port = params.get("port", None)
    path = params.get("path")
    hash_host = params.get("hash_host")
    state = params.get("state")
    # Find the ssh-keygen binary
    sshkeygen = module.get_bin_path("ssh-keygen", True)

    # Trailing newline in files gets lost, so re-add if necessary
    if key and key[-1] != "\n":
        key += "\n"

    if key is None and state != "absent":
        module.fail_json(msg="No key specified when adding a host")

    sanity_check(module, host, key, sshkeygen)

    found, replace_or_add, found_line, key = search_for_host_key(
        module, host, key, hash_host, path, sshkeygen
    )

    params["diff"] = compute_diff(path, found_line, replace_or_add, state, key)

    # We will change state if found==True & state!="present"
    # or found==False & state=="present"
    # i.e found XOR (state=="present")
    # Alternatively, if replace is true (i.e. key present, and we must change
    # it)
    if module.check_mode:
        module.exit_json(
            changed=replace_or_add or (state == "present") != found, diff=params["diff"]
        )

    # Now do the work.

    # Only remove whole host if found and no key provided
    if found and key is None and state == "absent":
        module.run_command([sshkeygen, "-R", host, "-f", path], check_rc=True)
        params["changed"] = True

    # Next, add a new (or replacing) entry
    if replace_or_add or found != (state == "present"):
        try:
            inf = open(path, "r")
        except IOError as e:
            if e.errno == errno.ENOENT:
                inf = None
            else:
                module.fail_json(msg="Failed to read %s: %s" % (path, str(e)))
        try:
            outf = tempfile.NamedTemporaryFile(mode="w+", dir=os.path.dirname(path))
            if inf is not None:
                for line_number, line in enumerate(inf):
                    if found_line == (line_number + 1) and (
                        replace_or_add or state == "absent"
                    ):
                        continue  # skip this line to replace its key
                    outf.write(line)
                inf.close()
            if state == "present":
                outf.write(key)
            outf.flush()
            module.atomic_move(outf.name, path)
        except (IOError, OSError) as e:
            module.fail_json(
                msg="Failed to write to file %s: %s" % (path, to_native(e))
            )

        try:
            outf.close()
        except:
            pass

        params["changed"] = True

    return params


# 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 enforce_state(module, params):
    """
    Add or remove key.
    """

    host = params["name"].lower()
    key = params.get("key", None)
    path = params.get("path")
    hash_host = params.get("hash_host")
    state = params.get("state")
    # Find the ssh-keygen binary
    sshkeygen = module.get_bin_path("ssh-keygen", True)

    if not key and state != "absent":
        module.fail_json(msg="No key specified when adding a host")

    if key and hash_host:
        key = hash_host_key(host, key)

    # Trailing newline in files gets lost, so re-add if necessary
    if key and not key.endswith("\n"):
        key += "\n"

    sanity_check(module, host, key, sshkeygen)

    found, replace_or_add, found_line = search_for_host_key(
        module, host, key, path, sshkeygen
    )

    params["diff"] = compute_diff(path, found_line, replace_or_add, state, key)

    # We will change state if found==True & state!="present"
    # or found==False & state=="present"
    # i.e found XOR (state=="present")
    # Alternatively, if replace is true (i.e. key present, and we must change
    # it)
    if module.check_mode:
        module.exit_json(
            changed=replace_or_add or (state == "present") != found, diff=params["diff"]
        )

    # Now do the work.

    # Only remove whole host if found and no key provided
    if found and not key and state == "absent":
        module.run_command([sshkeygen, "-R", host, "-f", path], check_rc=True)
        params["changed"] = True

    # Next, add a new (or replacing) entry
    if replace_or_add or found != (state == "present"):
        try:
            inf = open(path, "r")
        except IOError as e:
            if e.errno == errno.ENOENT:
                inf = None
            else:
                module.fail_json(msg="Failed to read %s: %s" % (path, str(e)))
        try:
            with tempfile.NamedTemporaryFile(
                mode="w+", dir=os.path.dirname(path), delete=False
            ) as outf:
                if inf is not None:
                    for line_number, line in enumerate(inf):
                        if found_line == (line_number + 1) and (
                            replace_or_add or state == "absent"
                        ):
                            continue  # skip this line to replace its key
                        outf.write(line)
                    inf.close()
                if state == "present":
                    outf.write(key)
        except (IOError, OSError) as e:
            module.fail_json(
                msg="Failed to write to file %s: %s" % (path, to_native(e))
            )
        else:
            module.atomic_move(outf.name, path)

        params["changed"] = True

    return params