R – NHibernate & Linq – “could not resolve property”

Mapping-File:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
               assembly="DaVinci"
               namespace="DaVinci.Domain">
  <class name="Waehrungskurs" table="WAEHRUNGSKURSE">
    <id name="Id" column="ID">
      <generator class="native" />
    </id>
    <property name="ISOCode" column="ISO" />
    <property name="AktuellerKurs" column="AKTKURS" />
    <property name="Kursart" column="KIND_OF_KURS" />
    <property name="WährungstabkurseId" column="WAEHRUNGSTABKURSE_ID" />
    <property name="Departure" column="DEPARTURE" />
    <property name="Datum" column="DATE_TIME" />
    <property name="RealerKurs" column="REALKURS" />
    <property name="Gültig" column="GUELTIG" />    
  </class>
</hibernate-mapping>

Domain.cs:
…
public virtual double AktuellerKurs { get; set; }
public virtual DateTime Datum { get; set;}
public virtual DateTime Gültig { get; set; }
public virtual int Id { get; set; }
public virtual string ISOCode { get; set; }
public virtual int Kursart { get; set; }
public virtual double RealerKurs { get; set; }
public virtual int WährungstabkurseId { get; set; }
…

I get a NHibernate.QueryException ("could not resolve property: Datum.Date of: DaVinci.Domain.Waehrungskurs") when i call the following function:

    public static Domain.Waehrungskurs GetByISOAndKursartAndDate(string isocode, int kursart, DateTime datum)
    {            

        return (from WK in session.Linq<DaVinci.Domain.Waehrungskurs>()                    
                where WK.ISOCode == isocode
                            &&
                      WK.Kursart == kursart
                            &&
                      WK.Datum.Date == datum.Date
                orderby WK.Id descending
                select WK).First();
    }

When i delete the search-condition "WK.Datum.Date == datum.Date", the function works fine.

Any ideas why the property cannot be found?