Say I wish to calculate $U(0,-1)$ which is $D_{-1/2}(-1)$ using the whitaker notation. According to http://dlmf.nist.gov/12.7.10, the identity is $$U(0,z)=\sqrt{\frac{z}{2\pi}}\mathcal{K}_{1/4}\left(\frac{1}{4}z^2\right)$$ However, the fact that z is under the square root sign gives a complex number. In wolfram alpha the value of $D_{-1/2}(-1)\approx1.83039$ which is more what I am expecting.
Am I missing something?