XML Deserialization of a date with an empty value

I'm getting a xml file from one vendor that has some "empty" dates like this:

<UpdatedOn/>
<DeletedOn/>

By doing a regular deserialization it fails with:

Inner Exception: System.FormatException: String was not recognized as a valid DateTime.

Any ideas how to deal with this ?

My fields are already marked for a default DateTime:

[System.Xml.Serialization.XmlElementAttribute(DataType="date")]
[System.ComponentModel.DefaultValueAttribute(typeof(System.DateTime), "1901-01-01")]
public System.DateTime UpdateOn{...}