Missing non-null postcondition on IEnumerable<T>.GetEnumerator()

Question

 

 

SortedList<string, string> map = new SortedList<string, string>();

 

 

var enumerator = map.GetEnumerator();

 

 

Contract.Assert(enumerator != null);

warning : CodeContracts: assert unproven

Should this not be a postcondition on the IEnumerable<T> interface, and as such automatically apply to all implementations?