Description

When serializing object containing property marked with [JsonExtensionData] attribute, serializer produces invalid JSON like this one: {"Id":1,{"nested":true}}

Reproduction Steps

Run the following code

var mix = new Mix
{
    Id = 1,
    Extra = new() { ["nested"] = true, }
};

var text = System.Text.Json.JsonSerializer.Serialize(mix);
Console.WriteLine(text);
// output {"Id":1,{"nested":true}}


public class Mix
{
    public int Id { get; set; }

    [System.Text.Json.Serialization.JsonExtensionData]
    public System.Text.Json.Nodes.JsonObject? Extra { get; set; }
}

Expected behavior

Correct JSON like {"Id":1,"nested":true} or at least valid JSON as if there was not [JsonExtensionData] attribute ({"Id":1,"Extra":{"nested":true}} ).

Actual behavior

Invalid JSON {"Id":1,{"nested":true}}

Regression?

Reproducible at least on .Net 6 to .Net 8

Known Workarounds

No response

Configuration

.Net 8
Windows 11
x64

Other information

No response