"Value '' could not be converted." error when clearing ComboBox selection

Question

I am trying to clear the selection in a ComboBox but get an error

"Value '' could not be converted."

The ComboBox's ItemSource is bound to a key-value pair list. The SelectedValue is the Key and the DisplayMemberPath is bound to the Value.

If the ItemSource is bound to an ordinary datatype, such as a string, and clear the selected value in ComboBox, this error doesn't occur. But I needed it as a Key-Value pair since its a lookup.

Suspect that the error could be because the key-value pair has no corresponding null entry or could not take a null value. Could this a bug in the framework. How to resolve this. Seen the blogs that says use Nullable value and do the conversion, but doesn't appear to be a good way to resolve this since an explicit conversion adapter have to be written. Is there a better way to resolve this.

Tried setting ItemSource binding to nullable value. But get a different error

'System.Nullable>' does not contain a definition for 'Key' and no extension method 'Key' accepting a first argument of type 'System.Nullable>' could be found (are you missing a using directive or an assembly reference?)

//XAML

<Combobox 
    Name="CityPostalCodeCombo"
    ItemsSource="{Binding CityList, TargetNullValue=''}"
    SelectedItem="{Binding PostalCode, UpdateSourceTrigger=PropertyChanged, TargetNullValue='', ValidatesOnDataErrors=True, NotifyOnValidationError=True, Mode=TwoWay}"
    SelectedValuePath="Key"
    DisplayMemberPath="Value"
    AllowNull="True"
    MinWidth="150"
    MaxHeight="50">

//Code:  View Model binding
private List<KeyValuePair<string, string>> cityList = GetCityList(); 

// City and postal code list
public List<KeyValuePair<string, string>> CityList
{
  get { return cityList; }
  set
  {
    if (value != cityList)
    {
      cityList = value;
      OnPropertyChanged("CityList");
    }
  }
}             

public KeyValuePair<string, string>? PostalCode
{
    get 
    { 
      return CityList.Where(s =>   s.Key.Equals(postalCode.Value)).First(); 
    }      
    set
    {
      if (value.Key != postalCode.Value)
      {
          postalCode.Value = value.Key;
          OnPropertyChanged("PostalCode");
      }
    }
}

// Populate Cities:
private static List<KeyValuePair<string, string>>GetCityList()
{   
    List<KeyValuePair<string, string>> cities = new List<KeyValuePair<string, string>>();   
    KeyValuePair<string, string> value = new KeyValuePair("94310", "Palo Alto");    
    cities.Add(value);  

    value = new KeyValuePair("94555", "Fremont");   
    cities.Add(value);  

    value = new KeyValuePair("95110", "San Jose");  
    cities.Add(value);  

    return cities;
}