C# JsonSerializer.Deserialize fails if property has null value despite JsonIgnoreCondition.WhenWritingNull

From an external webservice i receive either

// jsonWithConfig 
// property config is an object {}
{"config":{"c1":"All","c2":"is peachy"},"message":"We found a config object"} 

// jsonNoConfig
// property config is string with the value null
{"config":"null","message":"Config is null"} 

I want to deserialize the json into these types

public class WebResponse
{     
   [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]   
   public Config Config { get; set; }  
   public string Message { get; set; } 

   // i also tried dynamic but 
   //    a) this is not what i want
   //    b) it resulted in RuntimeBinderException
   // public dynamic Config { get; set; } 
}

public class Config
{
    public string C1 { get; set; }
    public string C2 { get; set; }
}

What have i tried?

From How to ignore properties with System.Text.Json i started with JsonSerializer.Deserialize from System.Text.Json

var options = new JsonSerializerOptions{
     DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
     PropertyNameCaseInsensitive = true
};
    
string jsonWithConfig = 
       {"config":{"c1":"All","c2":"is peachy"},"message":"We found a Config"}   
WebResponse webResponseWithConfig = 
            JsonSerializer.Deserialize<WebResponse>(jsonWithConfig, options);

This works for jsonWithConfig which is no surprise since the json can be deserilized to the type WebResponse.

What is the error?

I hoped that using JsonSerializerOptions.DefaultIgnoreCondition would work for jsonNoConfig = {"config":"null","message":"Config is null"}.
But deserialization of jsonNoConfig fails with DeserializeUnableToConvertValue

string jsonNoConfig = 
       {"config":"null","message":"Config is null"} 
WebResponse webResponseNoConfig = 
            JsonSerializer.Deserialize<WebResponse>(jsonNoConfig, options); 

Questions

• How can i deserialize jsonNoConfig?
• What must i do?

Update

MySkullCaveIsADarkPlace pointed out that config should have the value null and not "null". After changing this the code above works as expected.

But is there a way to handle null with quotation marks like {"config":"null", ...} as well?

Full stack trace

The stack trace inside linqpad shows this

at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type
propertyType)

at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader&
reader, Type typeToConvert, JsonSerializerOptions options, ReadStack&
state, T& value)

at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader&
reader, Type typeToConvert, JsonSerializerOptions options, ReadStack&
state, T& value)

at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object
obj, ReadStack& state, Utf8JsonReader& reader)

at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader&
reader, Type typeToConvert, JsonSerializerOptions options, ReadStack&
state, T& value)

at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader&
reader, Type typeToConvert, JsonSerializerOptions options, ReadStack&
state, T& value)

at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader&
reader, JsonSerializerOptions options, ReadStack& state)

at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan1 utf8Json, JsonTypeInfo jsonTypeInfo, Nullable1 actualByteCount)

at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1
json, JsonTypeInfo jsonTypeInfo)

at System.Text.Json.JsonSerializer.Deserialize[TValue](String json,
JsonSerializerOptions options) at UserQuery.Main(), line 13

LINQPad program

This linqpad-program has all the code needed

// Linqpad program
void Main()
{       
string jsonWithConfig = "{"config":{"c1":"All","c2":"is peachy"},"message":"We found a Config"}";
string jsonNoConfig   = "{"config":"null","Message":"Config is null"}";
    
var options = new JsonSerializerOptions{
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
        PropertyNameCaseInsensitive = true
};
        
WebResponse webResponseWithConfig = JsonSerializer.Deserialize<WebResponse>(jsonWithConfig, options);
webResponseWithConfig.Dump();
WebResponse webResponseNoConfig = JsonSerializer.Deserialize<WebResponse>(jsonNoConfig, options);   
    webResponseNoConfig.Dump();
}

// custom types
public class WebResponse
{     
   [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]   
   public Config Config { get; set; }     
   public string Message { get; set; }      
}

public class Config
{
    public string C1 { get; set; }
    public string C2 { get; set; }
}