  London penetration depth, $\lambda (T)$, was measured in single crystals of
K$_{1-x}$Na$_x$Fe$_2$As$_2$, $x$=0 and 0.07, down to temperatures of 50~mK,
$\sim T_c/50$. 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~K 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 multi-band scenarios with strongly different gap structure on
four sheets of the Fermi surface.
