Exception Querying with Linq - Nhibernate

var employee = session.Query<Employee>().Where(x => x.Name == "John").ToList();

public class Employee
    {
        public Guid UniqueId { get; set; }
        public string Name { get; set; }
    }

<xml>
<hibernate>
  <class>
    <id>
      <generator>
    </id>
    <property>
  </class>
</hibernate>

System.TypeInitializationException : The type initializer for 'NHibernate.Linq.NhRelinqQueryParser' threw an exception.
----> System.InvalidOperationException : Sequence contains more than one matching element
at NHibernate.Linq.NhRelinqQueryParser.Parse(Expression expression)
at NHibernate.Linq.NhLinqExpression.Translate(ISessionFactoryImplementor sessionFactory)
at NHibernate.Hql.Ast.ANTLR.ASTQueryTranslatorFactory.CreateQueryTranslators(String queryIdentifier, IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 filters, ISessionFactoryImplementor factory)
at NHibernate.Engine.Query.HQLExpressionQueryPlan.CreateTranslators(String expressionStr, IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
at NHibernate.Engine.Query.HQLExpressionQueryPlan..ctor(String expressionStr, IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
at NHibernate.Engine.Query.HQLExpressionQueryPlan..ctor(String expressionStr, IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
at NHibernate.Engine.Query.QueryPlanCache.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters)
at NHibernate.Impl.AbstractSessionImpl.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow)
at NHibernate.Impl.AbstractSessionImpl.CreateQuery(IQueryExpression queryExpression)
at NHibernate.Linq.NhQueryProvider.PrepareQuery(Expression expression, ref IQuery query, ref NhLinqExpression nhQuery)
at NHibernate.Linq.NhQueryProvider.Execute(Expression expression)
at NHibernate.Linq.NhQueryProvider.Execute(Expression expression)
at Remotion.Data.Linq.QueryableBase`1.GetEnumerator()
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Linq.Enumerable.ToList(IEnumerable`1 source)
at Fixture.Fixture.TestConfiguration() in Fixture.cs: line 28
--InvalidOperationException
at System.Linq.Enumerable.Single(IEnumerable`1 source, Func`2 predicate)
at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.Transformation.ExpressionTransformerRegistry.CreateDefault()
at NHibernate.Linq.NhRelinqQueryParser..cctor()

Sequence contains more than one matching element
at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.Transformation.Expression TransformerRegistry.CreateDefault() in :line 0
at NHibernate.Linq.NhRelinqQueryParser..cctor() in d:CSharpNHNH hibernatesrcNHibernateLinqNhRelinqQueryParser.cs:line 26