JamesNK/Newtonsoft.Json · error · FormatException

Integer string ' ' is not allowed.

Error message

Integer string '{0}' is not allowed.

What it means

ParseEnum (EnumUtils.cs:296-323), after determining the value parses as a number (char.IsDigit or sign at the start and Convert.ChangeType succeeds), checks the disallowNumber flag at EnumUtils.cs:317-320. If disallowNumber is true, it throws FormatException reporting the integer string. disallowNumber is set by callers that want to reject raw integer enum values (notably StringEnumConverter with AllowIntegerValues=false).

Solutions

1. Send enum values as their string member names in the JSON (e.g. "Active" rather than 3).
2. If integer values are acceptable, set the StringEnumConverter's AllowIntegerValues=true (and AllowNullValues if needed).
3. Pre-process the payload to map numbers to names before deserialization.
4. Use a custom JsonConverter that maps disallowed integers to a default member.

Example fix

// before: StringEnumConverter { AllowIntegerValues = false }, JSON = 3 -> throws
// after (option A): emit "Active" in JSON
// after (option B): new StringEnumConverter { AllowIntegerValues = true }

// If integers are disallowed, ensure the value is a name, not a number.
if (disallowNumber && value.Trim().TrimStart('-','+').All(char.IsDigit) && value.Trim().Length > 0)
    throw new FormatException($"Integer string '{value}' is not allowed.");
// or: set StringEnumConverter.AllowIntegerValues = true if integers are acceptable.

static bool LooksLikeInteger(string s) { s = s.Trim(); return s.Length > 0 && (char.IsDigit(s[0]) || s[0]=='-' || s[0]=='+') && s.TrimStart('-','+').All(char.IsDigit); }

try { return EnumUtils.ParseEnum(enumType, null, value, disallowNumber: true); } catch (FormatException ex) when (ex.Message.Contains("not allowed")) { /* map to a name or accept integers */ }

When it happens

Trigger: An enum field receives a numeric value (e.g. JSON `"status": 3` or the string "3") while the configured converter has AllowIntegerValues=false, so integer representations are forbidden.

Common situations: StringEnumConverter configured with AllowIntegerValues=false but the source emits integers (common with System.Text.Json interop or older producers); schema enforcing string-only enum values; serializers migrated from one that emitted numbers.

Related errors

• Enum name ' ' already exists on enum ' '.
• Enum type is not a set of flags.
• Could not resolve type
• Invalid date time handling value.
• Must specify valid information for parsing in the string.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/53859cd9dba8ada8. Report an issue: GitHub.

Appendix: source

Thrown at Src/Newtonsoft.Json/Utilities/EnumUtils.cs:319

                value = value.Trim();
                object? temp = null;

                try
                {
                    temp = Convert.ChangeType(value, underlyingType, CultureInfo.InvariantCulture);
                }
                catch (FormatException)
                {
                    // We need to Parse this as a String instead. There are cases
                    // when you tlbimp enums that can have values of the form "3D".
                    // Don't fix this code.
                }

                if (temp != null)
                {
                    if (disallowNumber)
                    {
                        throw new FormatException("Integer string '{0}' is not allowed.".FormatWith(CultureInfo.InvariantCulture, value));
                    }

                    return Enum.ToObject(enumType, temp);
                }
            }

            ulong result = 0;

            int valueIndex = firstNonWhitespaceIndex;
            while (valueIndex <= value.Length) // '=' is to handle invalid case of an ending comma
            {
                // Find the next separator, if there is one, otherwise the end of the string.
                int endIndex = value.IndexOf(EnumSeparatorChar, valueIndex);
                if (endIndex == -1)
                {
                    endIndex = value.Length;
                }

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