Using InvokeMember with WPF WebBrowser HTMLElement throws exception

Question

I'm trying to simulate a user typing a value into an "input" field on a website, and I'm getting very strange exception when trying to run InvokeMember. I tested several approaches and I found that most online documentation is outdated and simply doesn't work. Unfortunately the mostly suggested solution to my problem seems to throw a very strange exception:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: ''System.MarshalByRefObject.InvokeMember(string, System.Reflection.BindingFlags, System.Reflection.Binder, object[], System.Reflection.ParameterModifier[], System.Globalization.CultureInfo, string[])' is inaccessible due to its protection level'

This is an example code that recreates the problem:

MainWindow.xaml

<Window x:Class="Example.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:Example"
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800">
    <DockPanel>
        <WebBrowser x:Name="ie"/>
    </DockPanel>
</Window>

MainWindow.xaml.cs

using mshtml;
using System.ComponentModel;
using System.Threading;
using System.Windows;
using System.Windows.Navigation;

namespace Example
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            ie.LoadCompleted += (o, e) =>
            {
                BackgroundWorker worker = new BackgroundWorker();
                worker.DoWork += (wo, we) => { Thread.Sleep(5000); };
                worker.RunWorkerCompleted += (wo, we) => { DelayedLoadCompleted(); };
                worker.RunWorkerAsync();
            };

            ie.Navigate(@"https://google.com");
        }

        private void DelayedLoadCompleted()
        {
            HTMLDocument doc = (ie.Document as HTMLDocument);

            dynamic input = doc.getElementById("lst-ib");

            input.InvokeMember("onkeypress", new { Key = 65 }); //throws that excetpion
            input.InvokeMember("click"); //throws that excetpion
            input.click(); //works fine
        }
    }
}

The line input.InvokeMember("click"); throws that exception, but input.click(); works just fine.

What is the cause of that exception?