Dot Net Core Web API Return Response From Endpoint

I'm busy trying to create a very simple Dot Net Core Web API (Dot Net 5) and I've run into a strange issue where I cannot get the endpoint to return a response.

I've tried to use

await context.Response.WriteAsync("Hello World!");

as per the documentation but I'm getting the error

'HttpResponse' does not contain a definition for 'WriteAsync'

This is the full Startup.cs code

public class Startup {

    public Startup(IConfiguration configuration) {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services) { }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {

        app.UseRouting();

        app.UseEndpoints(endpoints => {
            endpoints.MapGet("/test", async context => {
                //Console.WriteLine("Exec Test");
                await context.Response.WriteAsync("Hello World!");
            });
        });

    }

}

I'm sure there's something I'm missing