ifftn yields complex numbers

I have a 3D array, called WT, with data of a traveling wave. The dimensions are y, x and time. My goal is to extract the forward and the backward traveling wave. Anybody have any ideas on how to do that?

What I have done so far is wt = fftn(WT) to convert the data into the f-k domain. I have ky, kx and frequency. Then I set all the values for kx<=0 to zero. Let's call this wF (forward traveling wave). I was hoping to simply do WF = ifftn(wF) to obtain the forward traveling wave in the time domain, however, the ifftn is giving me complex number. Any thoughts?