Picker Trigger Value Problem

Hi everyone !
In my screen there are picker and entry.If the user selects the picker value("Evli"), the entry will be visible. Here is my code in xaml ;

                <Picker Grid.Row="6"
                        Grid.Column="1"
                        WidthRequest="150"
                        TitleColor="Black"
                        Margin="10,10"
                        x:Name="pckrMaritalStatus"
                        Title="Medeni Hal">
                    <Picker.ItemsSource>
                        <x:Array Type="{x:Type x:String}">
                            <x:String>Evli</x:String>
                            <x:String>Bekar</x:String>
                        </x:Array>
                    </Picker.ItemsSource>
                </Picker>

                <Entry Grid.Row="8"
                       Grid.Column="0"
                       Grid.ColumnSpan="2"
                       Placeholder="Evlilik Yıldönümü"
                       Margin="10,10"
                       x:Name="txtYildonum"
                       PlaceholderColor="Black"
                       TextColor="#FA8E50"
                       IsVisible="false">
                    <Entry.Triggers>
                        <DataTrigger TargetType="Picker"
                                     Binding="{Binding Source={x:Reference pckrMaritalStatus}, Path=SelectedItem}"
                                     Value="Evli">
                            <Setter Property="IsVisible"
                                    Value="True"/>
                        </DataTrigger>
                    </Entry.Triggers>
                </Entry>

But I'm getting an error ; If, i get no errors when I delete the picker in the page.