JamesNK/Newtonsoft.Json · error · InvalidOperationException

Cannot pass a null parameter to the constructor.

Error message

Cannot pass a null parameter to the constructor.

What it means

Thrown inside JsonTypeReflector.GetCreator's parameterized lambda when one of the user-supplied constructor arguments is null. Because the code infers each parameter's Type via param.GetType(), a null parameter has no type to infer, so it cannot match a constructor overload.

Source

Thrown at Src/Newtonsoft.Json/Serialization/JsonTypeReflector.cs:280

        private static Func<object[]?, object> GetCreator(
            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors)]
            Type type)
        {
            Func<object>? defaultConstructor = (ReflectionUtils.HasDefaultConstructor(type, false))
                ? ReflectionDelegateFactory.CreateDefaultConstructor<object>(type)
                : null;

            return (parameters) =>
            {
                try
                {
                    if (parameters != null)
                    {
                        Type[] paramTypes = parameters.Select(param =>
                        {
                            if (param == null)
                            {
                                throw new InvalidOperationException("Cannot pass a null parameter to the constructor.");
                            }

                            return param.GetType();
                        }).ToArray();
                        ConstructorInfo? parameterizedConstructorInfo = type.GetConstructor(paramTypes);

                        if (parameterizedConstructorInfo != null)
                        {
                            ObjectConstructor<object> parameterizedConstructor = ReflectionDelegateFactory.CreateParameterizedConstructor(parameterizedConstructorInfo);
                            return parameterizedConstructor(parameters);
                        }
                        else
                        {
                            throw new JsonException("No matching parameterized constructor found for '{0}'.".FormatWith(CultureInfo.InvariantCulture, type));
                        }
                    }

                    if (defaultConstructor == null)

View on GitHub (pinned to 4f73e74372)

Solutions

1. Provide a non-null sentinel value (e.g. string.Empty, 0) in place of null where the parameter semantically allows a default.
2. Add an overload of the converter/naming-strategy constructor that does not take that parameter so null is never passed.
3. If the null is meaningful, write a custom JsonConverterAttribute that constructs the converter directly instead of relying on GetCreator's type inference.
4. Audit calls to CreateJsonConverterInstance / CreateNamingStrategyInstance to ensure no null elements in the args array.

Example fix

// before
[JsonConverter(typeof(DateConverter), null)]
public DateTime Created { get; set; }
// after: explicit overload or sentinel
[JsonConverter(typeof(DateConverter))]
public DateTime Created { get; set; }
public class DateConverter : JsonConverter { public DateConverter() {} ... }

if (args != null && args.Any(a => a == null)) throw new ArgumentException("null args not supported by GetCreator");

static bool HasNoNullArgs(object[] args) => args == null || args.All(a => a != null);

try { JsonTypeReflector.CreateJsonConverterInstance(type, args); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Cannot pass a null parameter")) {
    logger.Error(ex, "null constructor arg supplied to converter; supply a non-null value or add an overload."); throw;
}

When it happens

Trigger: Creating a JsonConverter or NamingStrategy instance via [JsonConverter(typeof(T), parameters)] or CreateJsonConverterInstance/CreateNamingStrategyInstance where one of the parameter objects is null.

Common situations: An attribute like [JsonConverter(typeof(MyConverter), null)] (rarely expressible but possible via programmatic attribute construction), a custom JsonConverterAttribute subclass that passes a null parameter, or code that builds converter/naming-strategy arguments dynamically and accidentally includes null.

Related errors

• No matching parameterized constructor found for '{0}'.
• No parameterless constructor defined for '{0}'.
• Error creating '{0}'.
• Could not get constructor for {0}.
• Unable to find default constructor for

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/2051f08aa8fed705. Report an issue: GitHub.