.net core - Middleware don't handle requests

I am having a problem with middleware in .Net Core 2. Middleware doesn't handle any coming requests. What I have implemented.

KeyValidatorMiddleware class:

public class KeyValidatorMiddleware
{
    private readonly RequestDelegate _next;

    public KeyValidatorMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {

        if (!context.Request.Headers.ContainsKey("api-key"))
        {
            context.Response.StatusCode = 401;
            await context.Response.WriteAsync("No API key found !");
            return;
        }

        await _next.Invoke(context);
    }
}

and in Startup.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseMvc();
    app.UseCors("MyPolicy");
    app.UseMiddleware<KeyValidatorMiddleware>();
}

And nothing works, What am i missing in order to make the middleware work?

You should move the UseMiddleware line to be nearer the top, middleware runs in order and it may stop at the MVC level.

For example:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseHsts();
    }

    app.UseMiddleware<KeyValidatorMiddleware>();

    app.UseHttpsRedirection();
    app.UseMvc();
    app.UseCors("MyPolicy");
}