How to bind the datacontext of a tabcontrol content to an instance of an viewmodel in a observablecollection
<p>I want to bind the content of my <code>TabControl</code> to an instance of my <code>StepViewModel</code> in a <code>ObservableCollection</code> Steps.</p>

<p>My ProcessViewModel:</p>

<pre><code>pubic class ProcessViewModel : ViewModelBase
{
    public ObservableCollection&lt;StepViewModel&gt; Steps
    {
        get { return _steps; }
        set { _steps = value; OnPropertyChanged("Steps"); }
    }
    public StepViewModel SelectedStep
    {
        // like above...
    }
}
</code></pre>

<p>My StepViewModel (DataContext should be the StepVMs in Steps of ProcessVM):</p>

<pre><code>public class StepViewModel : ViewModelBase
{
    public string Name { get {...} set {...} }
    public object Media { get {...} set {...} }
    //...
}
</code></pre>

<p>My TabControl (DataContext is ProcessViewModel):</p>

<pre><code>&lt;C1:C1TabControl
    ItemsSource="{Binding Steps}"
    SelectedItem="{Binding SelectedStep}"
    SelectionChanged="{tcSteps_OnSelectionChanged"&gt;
           &lt;C1:C1TabControl.ContentTemplate&gt;
                &lt;DataTemplate&gt;
                    &lt;local:StepView
                        DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type vmns:ProcessViewModel}}, Path=SelectedStep}"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch"&gt;
                    &lt;/local:StepView&gt;
                &lt;/DataTemplate&gt;
            &lt;/C1:C1TabControl.ContentTemplate&gt;
&lt;/C1:C1TabControl&gt;
</code></pre>

<p>The Compiler delivers the following output message:</p>

<blockquote>
  <p>"System.Windows.Data Error: 40 : BindingExpression path error:
  'SelectedStep' property not found on 'object' ''StepViewModel'
  (HashCode=32952144)'. BindingExpression:Path=SelectedStep;
  DataItem='StepViewModel' (HashCode=32952144); target element is
  'StepView' (Name='StepView'); target property is 'DataContext' (type
  'Object')"</p>
</blockquote>

<p>Does anyone know how I can solve the?
Thanks!</p>