What causes "Can not convert {0} to Guid." in JamesNK/Newtonsoft.Json?

Use the Nullable overload (Guid?)token["id"] so JSON null returns null.

JamesNK/Newtonsoft.Json · error · ArgumentException

Can not convert {0} to Guid.

Error message

Can not convert {0} to Guid.

What it means

Thrown by the explicit cast operator 'operator Guid(JToken)' (NON-nullable) when the token is not a Guid-compatible JValue. ArgumentException {0} is the resolved JTokenType; allowed types are GuidTypes ({String, Comment, Raw, Guid, Bytes}). Non-nullable overload, so a JSON null token ALSO throws. The library only accepts a GUID string, a Guid, or a 16-byte array; numeric/date/object tokens are rejected.

Source

Thrown at Src/Newtonsoft.Json/Linq/JToken.cs:1427

            if (v.Value is byte[] bytes)
            {
                return bytes;
            }

            throw new ArgumentException("Can not convert {0} to byte array.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
        }

        /// <summary>
        /// Performs an explicit conversion from <see cref="JToken"/> to <see cref="Guid"/>.
        /// </summary>
        /// <param name="value">The value.</param>
        /// <returns>The result of the conversion.</returns>
        public static explicit operator Guid(JToken value)
        {
            JValue? v = EnsureValue(value);
            if (v == null || !ValidateToken(v, GuidTypes, false))
            {
                throw new ArgumentException("Can not convert {0} to Guid.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
            }

            if (v.Value is byte[] bytes)
            {
                return new Guid(bytes);
            }

            return (v.Value is Guid guid) ? guid : new Guid(Convert.ToString(v.Value, CultureInfo.InvariantCulture)!);
        }

        /// <summary>
        /// Performs an explicit conversion from <see cref="JToken"/> to <see cref="Nullable{T}"/> of <see cref="Guid"/> .
        /// </summary>
        /// <param name="value">The value.</param>
        /// <returns>The result of the conversion.</returns>
        public static explicit operator Guid?(JToken? value)
        {
            if (value == null)

View on GitHub (pinned to 4f73e74372)

Solutions

Use the Nullable overload (Guid?)token["id"] so JSON null returns null.
Guard token["id"]?.Type is String or Guid before casting, and use Guid.TryParse on the string value.
If ids are now integers or ObjectIds, change the consuming type and parse accordingly.
Deserialize into a Guid? POCO property to absorb nulls cleanly.

Example fix

// before
Guid id = (Guid)payload["id"]; // throws on JSON null or integer id

// after
Guid id = Guid.TryParse((string)payload["id"]!, out var g) ? g : Guid.Empty;

Defensive patterns

Strategy: validation

Validate before calling

Guid id = Guid.TryParse((string?)payload["id"], out var g) ? g : Guid.Empty;

Type guard

static bool IsGuidLike(JToken? t) =>
    t is JValue v && (v.Type == JTokenType.Guid || v.Type == JTokenType.String || v.Type == JTokenType.Bytes);

Try / catch

Guid id;
try { id = (Guid)payload["id"]; }
catch (ArgumentException) { id = Guid.Empty; }

Prevention

Use the Nullable overload (Guid?) for fields that may be JSON null.
Always Guid.TryParse string nodes instead of blind casts (avoids FormatException too).
If ids became integers/ObjectIds, change the consuming type at the boundary.

When it happens

Trigger: Calling (Guid)token["id"] when the JValue Type is Integer, Float, Boolean, Date, Uri, TimeSpan, Null, Object, or Array, or the node is a JObject/JArray (EnsureValue null). A malformed GUID STRING passes ValidateToken then throws FormatException from new Guid(string), not this error.

Common situations: Id field changed from GUID to integer or to a Mongo ObjectId hex string of wrong length; null GUID in some records cast to non-nullable Guid; mis-indexed node returning an object.

Related errors

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/7141f5120c5bf967. Report an issue: GitHub.