Binding to internal ViewModel-Property

I have a UserControl with a ViewModel class as DataContext:

XAML

<UserControl x:Class="DotfuscatorTest.UserControl.View.UserControlView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" >    
<StackPanel>
  <TextBox Text="{Binding ViewModelProperty}"/>
</StackPanel>
</UserControl>

CodeBehind:

namespace DotfuscatorTest.UserControl.View
{
   using ViewModel;
   public partial class UserControlView
   {
      public UserControlView()
      {
         InitializeComponent();
         DataContext = new UserControlViewModel();         
      }
   }
}

ViewModel class:

namespace DotfuscatorTest.UserControl.ViewModel
{
   public class UserControlViewModel
   {
      private string viewModelProperty = "hello world";

      internal string ViewModelProperty
      {
        get { return viewModelProperty; }
        set { viewModelProperty = value; }
      }
   }
}

If I set the ViewModelProperty to public the binding works fine. But if I set the property to internal like above the binding fails (Binding error: property not found... ).

I thought an internal property is accessible like public in same assembly. Also I can access to the property from UserControl-codebehind without any problem:

{
...

((UserControlViewModel)DataContext).ViewModelProperty = "hallo viewmodel";

...

Any explenation for this behavior?

Thanks in advance, rhe1980