mgth/LittleBigMouse · error · FormatException

' ' is not a MAC address: expected twelve hexadecimal…

Error message

'${value}' is not a MAC address: expected twelve hexadecimal digits, optionally grouped by ':', '-' or '.'.

What it means

ParseMacAddress strips separators (':', '-', '.') and whitespace from the input, then copies hex digits into a stack buffer of 2*MacLength characters. It throws NotAMacAddress(macAddress) — with this message — when it encounters either a full buffer followed by another non-separator character, or a character that is not a hex digit. This is the too-many/invalid-character branch at line 30.

Solutions

1. Correct the MAC string to contain exactly 12 hex digits (0-9a-fA-F), optionally grouped by ':', '-' or '.', e.g. "AA:BB:CC:DD:EE:FF".
2. Strip any label or trailing text before passing the value: pass only the address itself, not a whole settings line.
3. Validate in config with a regex ^[0-9A-Fa-f]{2}([:-.]?[0-9A-Fa-f]{2}){5}$ before invoking the API.
4. Verify you are not passing an IPv4/IP-based identifier where a MAC is required.

Example fix

// before
var packet = WakeOnLan.CreateMagicPacket("AA:BB:CC:DD:EE:GG"); // throws
// after
var packet = WakeOnLan.CreateMagicPacket("AA:BB:CC:DD:EE:FF"); // valid hex

static bool LooksLikeMac(string? mac) =>
    !string.IsNullOrWhiteSpace(mac) &&
    System.Text.RegularExpressions.Regex.IsMatch(mac, "^[0-9A-Fa-f]{2}([:-. ]?[0-9A-Fa-f]{2}){5}$");

bool IsMacAddress(string? value) =>
    !string.IsNullOrWhiteSpace(value) &&
    value.Where(c => c != ':' && c != '-' && c != '.' && !char.IsWhiteSpace(c)).Count() == 12 &&
    value.Where(c => c != ':' && c != '-' && c != '.' && !char.IsWhiteSpace(c)).All(Uri.IsHexDigit);

try { var packet = WakeOnLan.CreateMagicPacket(mac); }
catch (FormatException e) when (e.Message.Contains("not a MAC address")) {
    log.LogError("Invalid WOL target MAC '{Mac}'", mac);
    ui.ShowFieldError(nameof(mac), "Expected 12 hex digits, optionally grouped by :, - or .");
}

When it happens

Trigger: Calling ParseMacAddress (or CreateMagicPacket) with a string containing 12+ hex digits plus an invalid 13th+ character, or any non-hex character (e.g. 'G', '_') in the middle, e.g. "AA:BB:CC:DD:EE:GG" or "AA-BB-CC-DD-EE-FF-01".

Common situations: User typo in the Wake-on-LAN target MAC in config; pasting a MAC with a trailing label ('MAC: AA:BB...'); using an IPv4 address or UUID instead of a MAC; copy errors from the TV/device settings screen.

Understand the failure class

Background: "invalid id" errors: invalid identifier format — why libraries reject IDs before lookup, and how to fix them — this error's family across 37 libraries.

Related errors

• Enter a valid projector IPv4 address.
• A Wi-Fi MAC address is required for VIDAA authentication.
• The network device does not advertise VIDAA remote control.
• Invalid VIDAA remote key.
• Enter a VIDAA picture-setting menu ID between 1 and 999.

AI-assisted analysis of mgth/LittleBigMouse@7a42f01d47 (2026-09-16). Data as JSON: /api/errors/73ae6c4464416819. Report an issue: GitHub.

Appendix: source

Thrown at LittleBigMouse.Plugins/LittleBigMouse.Plugin.Vcp/Networking/WakeOnLan.cs:30

    /// <summary>Length of every magic packet: 6 synchronisation bytes + 16 copies of the MAC.</summary>
    public const int MagicPacketLength = MacLength + 16 * MacLength;

    /// <summary>
    /// Reads the six address bytes from the usual notations: "aa:bb:cc:dd:ee:ff",
    /// "AA-BB-CC-DD-EE-FF", "aabb.ccdd.eeff" or twelve bare hexadecimal digits.
    /// </summary>
    /// <exception cref="FormatException">The value is not twelve hexadecimal digits.</exception>
    public static byte[] ParseMacAddress(string macAddress)
    {
        ArgumentNullException.ThrowIfNull(macAddress);

        Span<char> digits = stackalloc char[2 * MacLength];
        var count = 0;
        foreach (var character in macAddress)
        {
            if (character is ':' or '-' or '.' || char.IsWhiteSpace(character)) continue;
            if (count == digits.Length || !Uri.IsHexDigit(character)) throw NotAMacAddress(macAddress);
            digits[count++] = character;
        }
        if (count != digits.Length) throw NotAMacAddress(macAddress);

        return Convert.FromHexString(digits);
    }

    /// <summary>Builds the magic packet waking <paramref name="macAddress"/>.</summary>
    public static byte[] CreateMagicPacket(string macAddress)
    {
        var mac = ParseMacAddress(macAddress);

        var packet = new byte[MagicPacketLength];
        Array.Fill(packet, (byte)0xff, 0, MacLength);
        for (var offset = MacLength; offset < packet.Length; offset += MacLength) mac.CopyTo(packet, offset);
        return packet;
    }

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