Tuple<T1, T2, T3, T4, T5, T6, T7, T8> expects the last element to be a tuple itself. The docs say:

TRest: Any generic Tuple object that defines the types of the tuple's remaining components.

Also, the Tuple.Create overload that creates a tuple with 8 elements returns the following type: Tuple<T1,T2,T3,T4,T5,T6,T7,Tuple<T8>>