A key aspect of unconventional pairing by the antiferromagnetic
spin-fluctuation mechanism is that the superconducting energy gap must
have the opposite sign on different parts of the Fermi surface. Recent
observations of non-nodal gap structure in the heavy-fermion
were then very surprising, given that this material has long been
considered a prototypical example of a superconductor where the Cooper
pairing is magnetically mediated. Here we present a study of the effect
of controlled point defects, introduced by electron irradiation, on the
temperature-dependent magnetic penetration depth λ(T) in CeCu2Si2.
We find that the fully gapped state is robust against disorder,
demonstrating that low-energy bound states, expected for sign-changing
gap structures, are not induced by nonmagnetic impurities. This provides
bulk evidence for s++-wave superconductivity without sign reversal.