JamesNK/Newtonsoft.Json · error · ArgumentException

Object serialized to

Error message

Object serialized to {0}. JObject instance expected.

What it means

ArgumentException thrown by JObject.FromObject when the source object serializes to a JToken whose type is not Object (e.g. it produced an Array, String, Integer, etc.). FromObject requires the serialized form to be a JSON object; passing something that serializes to an array or primitive fails.

Solutions

1. Validate the input type before calling FromObject, or use JToken.FromObject and branch on token.Type.
2. If the source is a list, deserialize to JArray.FromObject instead.
3. Adjust or remove the JsonConverter that is altering the serialized shape.

Example fix

// before
JObject o = JObject.FromObject(myList); // serializes to array -> ArgumentException

// after
JToken token = JToken.FromObject(myList);
JObject o = token as JObject;
if (o == null) throw new InvalidOperationException($"Expected object, got {token.Type}");

JToken token = JToken.FromObject(o, serializer);
if (token.Type != JTokenType.Object)
    throw new ArgumentException($"Object serialized to {token.Type}; JObject expected.", nameof(o));
return (JObject)token;

static bool SerializesToObject(object o, JsonSerializer s) =>
    JToken.FromObject(o, s).Type == JTokenType.Object;

try { return JObject.FromObject(o, serializer); }
catch (ArgumentException ex) when (ex.Message.Contains("JObject instance expected"))
{
    // Source was an array/primitive; handle accordingly.
}

When it happens

Trigger: Calling JObject.FromObject on a List/IEnumerable/array (serializes to JArray), on a primitive/string (serializes to JValue), or on an object whose JsonConverter emits a non-object token. Also when a custom JsonSerializer/contract resolver changes the serialized shape.

Common situations: Passing a collection where a single object was expected; a converter that flattens an object to a string; wrapping logic that assumes every input is object-shaped.

Related errors

• Accessed JObject values with invalid key value
• Can not add to .
• Set JObject values with invalid key value
• Argument is not a JToken.
• array

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/76e7b96b65a4388d. Report an issue: GitHub.

Appendix: source

Thrown at Src/Newtonsoft.Json/Linq/JObject.cs:509

        {
            return FromObject(o, JsonSerializer.CreateDefault());
        }

        /// <summary>
        /// Creates a <see cref="JObject"/> from an object.
        /// </summary>
        /// <param name="o">The object that will be used to create <see cref="JObject"/>.</param>
        /// <param name="jsonSerializer">The <see cref="JsonSerializer"/> that will be used to read the object.</param>
        /// <returns>A <see cref="JObject"/> with the values of the specified object.</returns>
        [RequiresUnreferencedCode(MiscellaneousUtils.TrimWarning)]
        [RequiresDynamicCode(MiscellaneousUtils.AotWarning)]
        public new static JObject FromObject(object o, JsonSerializer jsonSerializer)
        {
            JToken token = FromObjectInternal(o, jsonSerializer);

            if (token.Type != JTokenType.Object)
            {
                throw new ArgumentException("Object serialized to {0}. JObject instance expected.".FormatWith(CultureInfo.InvariantCulture, token.Type));
            }

            return (JObject)token;
        }

        /// <summary>
        /// Writes this token to a <see cref="JsonWriter"/>.
        /// </summary>
        /// <param name="writer">A <see cref="JsonWriter"/> into which this method will write.</param>
        /// <param name="converters">A collection of <see cref="JsonConverter"/> which will be used when writing the token.</param>
        [RequiresUnreferencedCode(MiscellaneousUtils.TrimWarning)]
        [RequiresDynamicCode(MiscellaneousUtils.AotWarning)]
        public override void WriteTo(JsonWriter writer, params JsonConverter[] converters)
        {
            writer.WriteStartObject();

            for (int i = 0; i < _properties.Count; i++)
            {

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