A van der Waals gas is at a temperature exactly equal to its Boyle temperature. At moderate pressure, the compressibility factor is

At $T_B$ only the first-order term in $P$ (the second virial coefficient) vanishes. The third virial coefficient term, which for a van der Waals gas is positive ($C=b^2$), still contributes, so $Z$ is slightly above 1 once the pressure is appreciable. Boyle's law is therefore obeyed only approximately and over a limited range.