Calling an Ambiguous Error Using Linq

First of all, sorry for the excesive generic name classes. My employer is paranoid, and I know for sure that he roams this site. So, I have this code:

var fooObj = new MyFooClass()
{
    fooField1 = MyEnum.Value3, 
    fooField2 = false,
    fooField3 = false,
    fooField4 = otherEntity.OneCollection.ElementAt(0) as MyBarClass
}

where otherEntity.OneCollection is an ISet. ISet is an NHibernate collection class that implements IEnumerable. If I try to compile this code, I get this error:

Error 2     The call is ambiguous between the following methods or properties: 
'System.Linq.Enumerable.ElementAt<MyFirm.Blah.Blah.ClassyClass>    (System.Collections.Generic.IEnumerable<MyFirm.Blah.Blah.ClassyClass>, int)'
and 
'System.Linq.Enumerable.ElementAt<MyFirm.Blah.Blah.ClassyClass>(System.Collections.Generic.IEnumerable<MyFirm.Blah.Blah.ClassyClass>, int)'    

But if I remove the use of System.Linq at the beginning of the class and change the code to this:

var fooObj = new MyFooClass()
{
    fooField1 = MyEnum.Value3, 
    fooField2 = false,
    fooField3 = false,
    fooField4 = System.Linq.Enumerable
                     .ElementAt(otherEntity.OneCollection, 0) as MyBarClass
}

It compiles and works. (?) to check if OneCollection elements are removed for clarity)

Can someone explain this error to me?

In case of relevance: I am using Visual Studio 2008, targeting the .NET Framework 3.5 and using ReSharper 5.1.

. - , , IEnumerable , .