WPF validation rule preventing decimal entry in textbox?

I have a WPF textbox defined in XAML like this:

<Window.Resources>        

    <Style x:Key="textBoxInError" TargetType="{x:Type TextBox}">

        <Style.Triggers>

            <Trigger Property="Validation.HasError" Value="true">

                <Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>

            </Trigger>

        </Style.Triggers>

    </Style>

</Window.Resources>



<TextBox x:Name="upperLeftCornerLatitudeTextBox" Style="{StaticResource textBoxInError}">

    <TextBox.Text>

        <Binding Path="UpperLeftCornerLatitude" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">

            <Binding.ValidationRules>

                <local:LatitudeValidationRule ValidationStep="RawProposedValue"/>

            </Binding.ValidationRules>

        </Binding>

    </TextBox.Text>

</TextBox>

As you can see, my textbox is bound to a decimal property on my business object called UpperLeftCornerLatitude which looks like this:

private decimal _upperLeftCornerLongitude;

public decimal UpperLeftCornerLatitude

{

    get { return _upperLeftCornerLongitude; }

    set

    {

        if (_upperLeftCornerLongitude == value)

        {

            return;

        }



        _upperLeftCornerLongitude = value;

        OnPropertyChanged(new PropertyChangedEventArgs("UpperLeftCornerLatitude"));

    }

}

My user will be entering a latitude value into this textbox and in order to validate that entry, I've created a validation rule that looks like this:

public class LatitudeValidationRule : ValidationRule

{

    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)

    {

        decimal latitude;



        if (decimal.TryParse(value.ToString(), out latitude))

        {

            if ((latitude < -90) || (latitude > 90))

            {

                return new ValidationResult(false, "Latitude values must be between -90.0 and 90.0.");

            }

        }

        else

        {

            return new ValidationResult(false, "Latitude values must be between -90.0 and 90.0.");

        }



        return new ValidationResult(true, null);

    }

}

My textbox initially starts off empty and I have a breakpoint set at the beginning of my validation rule. I enter 1 in the textbox and when my debugger breaks inside of the validation rule, I can see that value = "1". So far so good. Now I continue running and enter a decimal point in the textbox (so we should have "1." now). Again, the debugger breaks inside of the validation rule and, as expected, value = "1.". If I step through the validation rule code, I see that it passes the latitude value check and returns the following:

new ValidationRule(true, null);

However, as soon as the validation rule returns and I step into the next line of code, I find myself on the first line of my UpperLeftCornerLatitude property setter. Mousing over value here reveals that it's a value of "1" instead of "1." as I would expect. So naturally when I continue running my code, I end up back in the textbox staring at a value of "1" instead of "1.". If I remove all of the breakpoints, the effect is that I can't seem to enter a decimal point in the textbox. Is there something obvious that I'm missing here that's causing my setter to end up with a value of "1" even though I have entered "1." in the textbox? Thanks very much!