GetField() returning null

I'm trying to set the IsReadOnly property via reflection but GetField() keep returning null. I have no idea why.

Here's my code:

class Foo
{
  [ReadOnly(true)]
    public int x { get; set; }
    public int y { get; set; }

    public void lol()
    {
         PropertyDescriptor descriptor = TypeDescriptor.GetProperties(this.GetType())["x"];
         ReadOnlyAttribute attr = (ReadOnlyAttribute)descriptor.Attributes[typeof(ReadOnlyAttribute)];
         attr.Dump();
         attr.GetType().GetField("IsReadOnly", BindingFlags.NonPublic | BindingFlags.Instance).Dump();
    }
}

output (on LINQPAD):

Why is this returning null if the property is in there (as shown in the image)?

typeof(ReadOnlyAttribute).GetField("IsReadOnly", BindingFlags.NonPublic | BindingFlags.Instance)

doesn't works either.