ComboBox which is bound to a db table should initially be blank - xaml

Question

I have a WPF combobox control bound to a edm field in a database table. This works fine except that it enters the first value in the control when it starts which is not wanted. Heres the xaml
<ComboBox x:Name="cbMeasure"
Width="104"
ItemsSource="{Binding Source={StaticResource ddMeasureViewSource}}"
DisplayMemberPath="Measure"
IsSynchronizedWithCurrentItem="True"
SelectedValuePath="Measure"
SelectedValue="{Binding Measure1}"/>
If I just hard code the control it doesn't put the first value in the variable. Heres what that xaml looks like
<ComboBox x:Name="cbMeasure" Width="104" Text="{Binding Measure1}">
<TextBlock Text="one"/>
<TextBlock Text="two"/>
<TextBlock Text="three"/>
<TextBlock Text="four"/>
</ComboBox>
What do I have to do to make the database bound combobox start with an empty value the way the textbox combobox does? This is a problem as it puts the first value in the SelectedValue bound to a variable (Measure1).
The db table ddMeasure looks like:
RID Measure
--- -------
1 One
2 Two
3 Three
4 Four
so "One" in put into the ComboBox selection and Measure1 variable is populated as well.

Answer 1

OK I found the solution to this. You have to set the property IsSynchronizedWithCurrentItem to false which I thought I had tried but I think I just removed that property and the default must be true. So the xaml looks like
<ComboBox x:Name="cbMeasure"
Width="104"
ItemsSource="{Binding Source={StaticResource ddMeasureViewSource}}"
DisplayMemberPath="Measure"
IsSynchronizedWithCurrentItem="False"
SelectedValuePath="Measure"
SelectedValue="{Binding Measure1}"/>
Hope this helps others who try to find info on this type of binding.