FromHeader Asp.NET Core binding to default value

I'm testing Asp.Net core WebApi with the following basic controller:

[Route("test")]
public class TestController
{
    [Route("mult")]
    [HttpGet]
    public int Multiply(int x, int y)
    {
        return x * y;
    }
}

Then, in Fiddler, I send the following request:

And, for some reason, the response is 0. Both x and y are binded to the default integer value when entering the method.

I also tried:

[Route("test")]
public class TestController
{
    [Route("mult")]
    [HttpGet]
    public int Multiply([FromHeader]int x, [FromHeader]int y)
    {
        return x * y;
    }
}

But the result is the same. What am I doing wrong?