Why do I get this InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Http.RequestDelegate'?

I've just upgraded my ASP.NET Core WebApi project from .NET Core 2.2 to 3.1.

I've fixed up all of the compile-time errors, upgraded my Nuget packages, and I can now run the app.

However, When I call Build() on my IHostBuilder, I get the following exception:

InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Http.RequestDelegate' while attempting to activate 'MyProject.Api.Middleware.ExceptionHandlerMiddleware'.

The Middleware it's referring to is pretty standard.

ExceptionHandlerMiddleware.cs

public class ExceptionHandlerMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger<ExceptionHandlerMiddleware> _logger;

    public ExceptionHandlerMiddleware(RequestDelegate next, ILogger<ExceptionHandlerMiddleware> logger)
    {
        _logger = logger;
        _next = next;
    }

    public async Task InvokeAsync(HttpContext httpContext)
    {
        // redacted
    }
}

The rest of my app initialisation is fairly standard and I didn't change much going from 2.2 to 3.1 (2.2 was working).

I did change from services.AddMvc() to services.AddControllers().

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    private static IHostBuilder CreateHostBuilder(string[] args)
    {
        return Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(builder =>
            {
                builder.UseSerilog().UseStartup<Startup>();
            })
            .ConfigureLogging((context, logging) =>
            {
                logging
                .AddConfiguration(context.Configuration.GetSection("Logging"))
                .AddConsole()
                .AddDebug();
            });
    }
}

It's also worth mentioning that the ConfigureServices() method in Startup.cs is being called and runs fine, but Configure() never runs. The Build() method always kills the app before it gets to Configure().

My Startup's Configure method signature looks like this:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)