coordination compounds - Why is tetracyanonickelate square planar?

Why is tetracyanonickelate, $\ce{[Ni(CN)4]^{2-}}$ square planar?

• $\ce{CN}$, for being a strong field, would not follow Hund's rule.



• the central atom $\ce{Ni}$ would have a +2 charge so its configuration would change from $\ce{3d^8 4s^2}$ to $\ce{3d^8 4s^0}$


• Using the statements above and using crystal field theory I get its hybridization as $\ce{dsp^3}$, which suggests that its shape must be trigonal bipyramidal while it is not.