homework - Hybridization of tetraamminecopper(II) sulfate

$\ce{Cu^2+}$ has 9 electrons and a d-orbital and is almost completely filled (except 1 electron vacant) and $\ce{NH3}$ donates an electron eventually forming $\mathrm{sp^3}$ hybrid orbitals.

But it has a square planar geometry hence completely contradicting the above reasoning. Please explain what the hybridization is?