JamesNK/Newtonsoft.Json · error · ArgumentException

A name is required when setting property name state.

Error message

A name is required when setting property name state.

What it means

Thrown on the synchronous path (JsonWriter.cs) inside InternalWriteToken when writing a JsonToken.PropertyName whose value is not a string. JSON property names are required to be strings; a non-string value is rejected with ArgumentException to prevent malformed output. Identical guard to the async variant.

Source

Thrown at Src/Newtonsoft.Json/JsonWriter.cs:1710

        /// <param name="token">The <see cref="JsonToken"/> being written.</param>
        /// <param name="value">The value being written.</param>
        protected void SetWriteState(JsonToken token, object value)
        {
            switch (token)
            {
                case JsonToken.StartObject:
                    InternalWriteStart(token, JsonContainerType.Object);
                    break;
                case JsonToken.StartArray:
                    InternalWriteStart(token, JsonContainerType.Array);
                    break;
                case JsonToken.StartConstructor:
                    InternalWriteStart(token, JsonContainerType.Constructor);
                    break;
                case JsonToken.PropertyName:
                    if (!(value is string s))
                    {
                        throw new ArgumentException("A name is required when setting property name state.", nameof(value));
                    }

                    InternalWritePropertyName(s);
                    break;
                case JsonToken.Comment:
                    InternalWriteComment();
                    break;
                case JsonToken.Raw:
                    InternalWriteRaw();
                    break;
                case JsonToken.Integer:
                case JsonToken.Float:
                case JsonToken.String:
                case JsonToken.Boolean:
                case JsonToken.Date:
                case JsonToken.Bytes:
                case JsonToken.Null:
                case JsonToken.Undefined:

View on GitHub (pinned to 4f73e74372)

Solutions

1. Pass only string values to WritePropertyName.
2. Ensure PropertyName tokens in any JToken/WriteToken pipeline carry a string value.
3. Stringify non-string keys before writing (e.g. key.ToString()).

Example fix

// before
writer.WriteToken(JsonToken.PropertyName, 42);

// after
writer.WritePropertyName("42");

if (value is string name)
{
    writer.WritePropertyName(name);
}

static bool IsValidPropertyName(object value) => value is string;

When it happens

Trigger: Calling WritePropertyName with a non-string, or WriteToken/WriteTokenAsync routing a PropertyName token whose value is not a string; programmatic token construction with non-string names.

Common situations: Custom writers/token builders boxing numbers or objects as property names; dictionary serialization with non-string keys not pre-stringified; copy/paste of tokens from a non-conforming source.

Related errors

• A name is required when setting property name state.
• Unexpected value type when writing binary: {0}
• Expected Bytes but got {0}.
• Unexpected value when converting date. Expected DateTime or
• Expected date object value.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/44fb3acf877bcc54. Report an issue: GitHub.