London penetration depth, lambda(T), was measured in single crystals of K1-xNaxFe2As2, x = 0 and 0.07, down to temperatures of 50 mK, similar to T-c/50. Isovalent substitution of Na for K significantly increases impurity scattering, with rho(T-c) rising from 0.2 to 2.2 mu Omega cm, and leads to a suppression of T-c from 3.5 to 2.8 K. At the same time, a close to T-linear Delta lambda(T) in pure samples changes to almost T-2 in the substituted samples. The behavior never becomes exponential as expected for the accidental nodes, as opposed to T-2 dependence in superconductors with symmetry imposed line nodes. The superfluid density in the full temperature range follows a simple clean and dirty d-wave dependence, for pure and substituted samples, respectively. This result contradicts suggestions of multiband scenarios with strongly different gap structure on four sheets of the Fermi surface.