Why isn't my DataTemplate applied automatically

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.NET

XAML

WPF

DataTemplate

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Hi there!
Today I have another question I cannot find a solution for.
I have the following data templates in the Ressources-Section of my window:

<DataTemplate DataType="x:Type crosswordFields:EmptyField">
            <TextBlock MouseDown="FieldSelected" Width="20" Height="20" Tag="{Binding Tag}" Text="-">
            </TextBlock>
        </DataTemplate>
        <DataTemplate x:Key="BuildCrosswordDataTemplate">
            <ListView ItemsSource="{Binding}">
                <ListView.ItemsPanel>
                    <ItemsPanelTemplate>
                        <StackPanel Orientation="Horizontal"></StackPanel>
                    </ItemsPanelTemplate>
                </ListView.ItemsPanel>
            </ListView>
        </DataTemplate>

The first would be the template for my data structure i want to show in my ListView, the second one is the template for the ListView itself, which looks like the following:

<ListView Grid.Row="0" Name="lvBuildCrossword" ItemTemplate="{StaticResource BuildCrosswordDataTemplate}"></ListView>

As you can see, this is a ListView which contains further ListView's, but I don't think that that matters.
My problem now is, instead of showing a grid of TextBlocks which show an "-", i get a grid filled with Blocks containing "Crossword.CrosswordFields.EmptyField". Why isn't WPF using the DataTemplate defined for DataType="x:Type crosswordFields:EmptyField" but the default visualization via ToString()?

I hope someone can point me out.
Greets,
Markus