iOfficeAI/OfficeCLI · error · ArgumentException

{propName} contains the XML-illegal noncharacter U+{(int)c:X

Error message

{propName} contains the XML-illegal noncharacter U+{(int)c:X4} at position {i}.

What it means

Thrown by ValidateXmlText when the value contains U+FFFE or U+FFFF. These are Unicode noncharacters explicitly disallowed in XML 1.0 character data. (U+FFFE is also a byte-order-marker noncharacter; both are reserved and never legal text content.)

Source

Thrown at src/officecli/Core/ParseHelpers.cs:869

                throw new ArgumentException(
                    $"{propName} contains XML-illegal control character U+{(int)c:X4} at position {i}. " +
                    "Allowed control chars: \\t, \\n, \\r" +
                    (allowSoftBreakChar ? ", \\v." : "."));
            // UTF-16 surrogates only valid in pairs (high then low). A lone
            // half is illegal in XML 1.0 character data.
            if (char.IsHighSurrogate(c))
            {
                if (i + 1 >= value.Length || !char.IsLowSurrogate(value[i + 1]))
                    throw new ArgumentException(
                        $"{propName} contains an unpaired high surrogate U+{(int)c:X4} at position {i}. Use a complete UTF-16 surrogate pair.");
                i++; // skip the matched low surrogate
                continue;
            }
            if (char.IsLowSurrogate(c))
                throw new ArgumentException(
                    $"{propName} contains an unpaired low surrogate U+{(int)c:X4} at position {i}. Use a complete UTF-16 surrogate pair.");
            if (c == 0xFFFE || c == 0xFFFF)
                throw new ArgumentException(
                    $"{propName} contains the XML-illegal noncharacter U+{(int)c:X4} at position {i}.");
        }
    }
}

View on GitHub (pinned to 1ced45e900)

Solutions

1. Remove U+FFFE and U+FFFF from the string before setting the value.
2. Fix the upstream decoding that produced a U+FFFE (often a wrong byte order / misread BOM).
3. Replace sentinel characters with a valid placeholder before submission.

Example fix

// before
var text = "data\uFFFF";
// after
var text = "data";          // U+FFFF removed

static string RemoveXmlNoncharacters(string s)
    => s.Replace("\uFFFE", string.Empty).Replace("\uFFFF", string.Empty);

When it happens

Trigger: Setting any text value that contains the literal U+FFFE or U+FFFF character — e.g. binary/marker data decoded into a string, or test fixtures that include these code points.

Common situations: A BOM noncharacter (U+FFFE) left after mis-decoding bytes; sentinel/filler values from another system that uses U+FFFF as a 'no value' marker; fuzz-test inputs.

Related errors

• {propName} contains an unpaired high surrogate U+{(int)c:X4}
• {propName} contains an unpaired low surrogate U+{(int)c:X4}
• {propName} contains XML-illegal control character U+{(int)c:
• Invalid '{propertyName}' value '{value}': degrees must be in
• Invalid color value: '{value}'. Expected 6-digit hex RGB (e.

AI-assisted analysis of iOfficeAI/OfficeCLI@1ced45e900 (2026-08-13). Data as JSON: /api/errors/ede5e56f93364b20. Report an issue: GitHub.