microsoft/autogen · error · ArgumentNullException

Value cannot be null. (Parameter 'param.ParameterType')

Error message

Value cannot be null. (Parameter 'param.ParameterType')

What it means

ArgumentNullException ('param.ParameterType') thrown by ToChatTool when a function parameter has a null ParameterType. The extension must call JsonSchemaBuilder.FromType(param.ParameterType), which requires a concrete System.Type to generate the parameter's JSON schema.

Source

Thrown at dotnet/src/AutoGen.OpenAI/Extension/FunctionContractExtension.cs:31

{
    /// <summary>
    /// Convert a <see cref="FunctionContract"/> to a <see cref="ChatTool"/> that can be used in gpt funciton call.
    /// </summary>
    /// <param name="functionContract">function contract</param>
    /// <returns><see cref="ChatTool"/></returns>
    public static ChatTool ToChatTool(this FunctionContract functionContract)
    {
        var requiredParameterNames = new List<string>();
        var propertiesSchemas = new Dictionary<string, JsonSchema>();
        var propertySchemaBuilder = new JsonSchemaBuilder().Type(SchemaValueType.Object);
        foreach (var param in functionContract.Parameters ?? [])
        {
            if (param.Name is null)
            {
                throw new InvalidOperationException("Parameter name cannot be null");
            }

            var schemaBuilder = new JsonSchemaBuilder().FromType(param.ParameterType ?? throw new ArgumentNullException(nameof(param.ParameterType)));
            if (param.Description != null)
            {
                schemaBuilder = schemaBuilder.Description(param.Description);
            }

            if (param.IsRequired)
            {
                requiredParameterNames.Add(param.Name);
            }

            var schema = schemaBuilder.Build();
            propertiesSchemas[param.Name] = schema;

        }
        propertySchemaBuilder = propertySchemaBuilder.Properties(propertiesSchemas);
        propertySchemaBuilder = propertySchemaBuilder.Required(requiredParameterNames);

        var option = new System.Text.Json.JsonSerializerOptions()

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Always set ParameterType when constructing FunctionParameter (e.g. typeof(string))
2. Prefer TypeReflector.GetFunctionContract so types come from real method metadata
3. Validate the contract (null-check Name and ParameterType for every parameter) before calling ToChatTool

Example fix

// before
new FunctionParameter { Name = "city", ParameterType = null }
// ToChatTool -> ArgumentNullException

// after
new FunctionParameter { Name = "city", ParameterType = typeof(string), IsRequired = true }

foreach (var p in contract.Parameters ?? [])
{
    if (p.ParameterType is null) throw new ArgumentException($"Parameter '{p.Name}' of '{contract.Name}' has no type");
}

static bool ContractHasTypedParameters(FunctionContract c) => (c.Parameters ?? []).All(p => p.ParameterType is not null);

catch (ArgumentNullException ex) when (ex.ParamName == "param.ParameterType") { /* set ParameterType and rebuild tool */ }

When it happens

Trigger: A FunctionParameter with ParameterType == null inside a FunctionContract passed to ToChatTool — typically from reflection over generic/obfuscated code or hand-constructed contracts missing the type.

Common situations: Hand-built FunctionContract objects; custom TypeReflector replacements; methods with dynamic/COM parameters where reflection yields null types; partially-initialized contract objects.

Related errors

• Parameter name cannot be null
• Value cannot be null. (Parameter 'functionContract.Name')
• Please set OPENAI_API_KEY environment variable.
• Please set OPENAI_API_KEY environment variable.
• Please set OPENAI_API_KEY environment variable.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/f320fbdc41b88edd. Report an issue: GitHub.