JamesNK/Newtonsoft.Json · error · ArgumentException

Set JObject values with invalid key value: {0}. Object prope

Error message

Set JObject values with invalid key value: {0}. Object property name expected.

What it means

ArgumentException thrown by the JObject this[object key] setter when the supplied key is non-null but not a string. Setting a property via the object indexer requires a string property name; any other key type is rejected.

Source

Thrown at Src/Newtonsoft.Json/Linq/JObject.cs:350

        {
            get
            {
                ValidationUtils.ArgumentNotNull(key, nameof(key));

                if (!(key is string propertyName))
                {
                    throw new ArgumentException("Accessed JObject values with invalid key value: {0}. Object property name expected.".FormatWith(CultureInfo.InvariantCulture, MiscellaneousUtils.ToString(key)));
                }

                return this[propertyName];
            }
            set
            {
                ValidationUtils.ArgumentNotNull(key, nameof(key));

                if (!(key is string propertyName))
                {
                    throw new ArgumentException("Set JObject values with invalid key value: {0}. Object property name expected.".FormatWith(CultureInfo.InvariantCulture, MiscellaneousUtils.ToString(key)));
                }

                this[propertyName] = value;
            }
        }

        /// <summary>
        /// Gets or sets the <see cref="JToken"/> with the specified property name.
        /// </summary>
        /// <value></value>
        public JToken? this[string propertyName]
        {
            get
            {
                ValidationUtils.ArgumentNotNull(propertyName, nameof(propertyName));

                JProperty? property = Property(propertyName, StringComparison.Ordinal);

View on GitHub (pinned to 4f73e74372)

Solutions

1. Ensure the key is a string before assignment: jObject[(string)key] = value.
2. Use the strongly-typed this[string propertyName] setter.
3. Validate the key type upstream if it comes from untyped data.

Example fix

// before
jObject[userKey] = value; // userKey is int/object

// after
jObject[(string)userKey] = value;

static void SetByKey(JObject obj, object key, JToken? value)
{
    if (key is not string name)
        throw new ArgumentException("Key must be a string property name.", nameof(key));
    obj[name] = value;
}

static bool IsPropertyNameKey(object? key) => key is string;

try { jObject[key] = value; }
catch (ArgumentException ex) when (ex.Message.Contains("invalid key value"))
{
    jObject[Convert.ToString(key, CultureInfo.InvariantCulture)!] = value;
}

When it happens

Trigger: Assigning jObject[someInt] = value, jObject[guid] = value, or any non-string key on the object indexer setter — typically when a loosely-typed key variable holds a non-string.

Common situations: Generic code that writes into JObject via object keys; reflection/dynamic dispatch where the key type is not enforced; mixing array-style and object-style indexing.

Related errors

• Accessed JObject values with invalid key value: {0}. Object
• Can not add {0} to {1}.
• Object serialized to {0}. JObject instance expected.
• Argument is not a JToken.
• New item to be added to collection must be compatible with {

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/c9ee5036e46a0a40. Report an issue: GitHub.