Reading RSS in .NET Core 3.1 is not working

I have very simple .NET Core Web Api code that worked before in .NET Core 2.2 but stopped working after upgrading to 3.1

[HttpGet]
[Route("api/news")]
public IEnumerable<SyndicationItem> GetNews(string source)
{
    var feed = SyndicationFeed.Load(XmlReader.Create(source));
    return feed.Items;
}

I can invoke it by navigating to: https://localhost:5001/api/news?source=http://feeds.bbci.co.uk/news/rss.xml

In debug I can see it gets items just fine, but throws this error returning them, I don't understand why it stopped working after I upgraded to 3.1 and how to fix it

please advice, thanks!

System.NotSupportedException: The collection type 'System.Collections.Generic.Dictionary`2[System.Xml.XmlQualifiedName,System.String]' on 'System.ServiceModel.Syndication.SyndicationItem.AttributeExtensions' is not supported. at System.Text.Json.JsonClassInfo.GetElementType(Type propertyType, Type parentType, MemberInfo memberInfo, JsonSerializerOptions options) at System.Text.Json.JsonClassInfo.CreateProperty(Type declaredPropertyType, Type runtimePropertyType, Type implementedPropertyType, PropertyInfo propertyInfo, Type parentClassType, JsonConverter converter, JsonSerializerOptions options) at System.Text.Json.JsonClassInfo.AddProperty(Type propertyType, PropertyInfo propertyInfo, Type classType, JsonSerializerOptions options) at System.Text.Json.JsonClassInfo..ctor(Type type, JsonSerializerOptions options) at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type classType) at System.Text.Json.JsonClassInfo.get_ElementClassInfo() at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 originalWriterDepth, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state) at System.Text.Json.JsonSerializer.WriteAsyncCore(Stream utf8Json, Object value, Type inputType, JsonSerializerOptions options, CancellationToken cancellationToken) at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding) at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|29_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters() --- End of stack trace from previous location where exception was thrown --- at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync() --- End of stack trace from previous location where exception was thrown --- at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope) at Microsoft.AspNetCore.Routing.EndpointMiddleware.g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger) at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

The default JSON serializer in .NET Core 3.x is changed to System.Text.Json. In this case, you may need to specify to use Newtonsoft.Json as the default serializer (the default serializer in older versions of .NET Core).

In Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers().AddNewtonsoftJson();

    // other settings in ConfigureServices...
}