In Frege's logical perfect language (Begriffsschrift) every well-formed expression must have a reference (Bedeutung).
The Bedeutung of an expression is the actual thing corresponding to it.
The Sinn of an expression, however, is the “mode of presentation” or cognitive content associated with the expression in virtue of which the Bedeutung is picked out.
As showed by Klement's example above, in "natural" language, we use expressions without reference, like : "the least rapidly converging series". Of course, we can "understand" them; so they must have sense.
But if the sense of an expression is what enables us to "pick out" its reference, what is the "role" of sense if there is not a reference ?
So, at least in a "logical perfect language" :
no reference, no sense.