  The influence of the isospin-breaking $\pi^0\!\!-\!\eta\!-\!\eta'$ mixing
effect on the $CP$-asymmetries of $B\to K\pi$ processes is examined for the
first time. It is found that this mixing effect brings a large uncertainty both
to the $CP$-asymmetry sum rule of $B\to K\pi$ processes and the $CP$-asymmetry
difference of the $B^+\to K^+\pi^0$ and $B^0\to K^+\pi^-$, obscuring the
significance of the $K\pi$-puzzle. This uncertainty can be so large that it is
even possible to explain the $K\pi$-puzzle by the $\pi^0\!\!-\!\eta\!-\!\eta'$
mixing effect {\it alone}.
