microsoft/autogen · error · JsonException

Value was null.

Error message

Value was null.

What it means

JsonPropertyNameEnumConverter<T>.Read deserializes Mistral API enums by matching the JSON string against each enum field's [JsonPropertyName] attribute. System.Text.Json guarantees the token is a string for this converter only when the JSON shape matches the contract; a null/absent value where a string is expected surfaces as this JsonException.

Source

Thrown at dotnet/src/AutoGen.Mistral/Converters/JsonPropertyNameEnumConverter.cs:15

// Copyright (c) Microsoft Corporation. All rights reserved.
// JsonPropertyNameEnumConverter.cs

using System;
using System.Reflection;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace AutoGen.Mistral;

internal sealed class JsonPropertyNameEnumConverter<T> : JsonConverter<T> where T : struct, Enum
{
    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        string value = reader.GetString() ?? throw new JsonException("Value was null.");

        foreach (var field in typeToConvert.GetFields())
        {
            var attribute = field.GetCustomAttribute<JsonPropertyNameAttribute>();
            if (attribute?.Name == value)
            {
                return (T)Enum.Parse(typeToConvert, field.Name);
            }
        }

        throw new JsonException($"Unable to convert \"{value}\" to enum {typeToConvert}.");
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        var field = value.GetType().GetField(value.ToString());
        var attribute = field?.GetCustomAttribute<JsonPropertyNameAttribute>();

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Log the raw JSON response body to find which enum field arrived as null.
2. Update the AutoGen.Mistral package if the API contract changed (newer models mark such fields optional).
3. Pre-validate payloads: reject/null-check response JSON before deserialization when the field must exist.
4. Report the field name upstream so the converter/model can be made null-tolerant.

try { var completion = JsonSerializer.Deserialize<ChatCompletionResponse>(json); } catch (JsonException) { /* log raw json, reject payload */ }

When it happens

Trigger: A Mistral API response contains an explicit JSON null (or a malformed payload handled as null) where an enum decorated with this converter is expected — e.g. a null tool_call finish reason or null role field in a chat completion.

Common situations: Mistral API updates that make previously-required enum fields nullable; deserializing error payloads or partial streaming chunks with missing enum fields; schema drift between the SDK models and the live API.

Related errors

• Unable to convert "{value}" to enum {typeToConvert}.
• Failed to deserialize response
• choice.Message.Content
• choice.Message.ToolCalls
• Exception of type 'System.Text.Json.JsonException' was throw

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/f9e6eb03ed29f567. Report an issue: GitHub.