  Josephson current between two superconductors provides a phase sensitive tool
for probing their pairing symmetries. Here we fabricate and study
experimentally high-quality Josephson junctions between a conventional s-wave
superconductor Nb and a multi-band iron-pnictide Ba$_{1-x}$Na$_x$Fe$_2$As$_2$.
Junctions exhibit a large enough critical current density to preclude the
d-wave symmetry of the order parameter in the pnictide. However, the $I_cR_n$
product is very small $\simeq 3~\mu$V, which is not consistent with the
sign-preserving $s_{++}$ symmetry either. We argue that the small $I_cR_n$
value along with its unusual temperature dependence provide evidence for the
$s_{\pm}$ symmetry of the order parameter in Ba$_{1-x}$Na$_x$Fe$_2$As$_2$. We
conclude that it is the phase sensitivity of our junctions that leads to an
almost complete (bellow a sub-percent) cancellation of opposite supercurrents
from the sign-reversal $s_{\pm}$ bands in the pnictide.
