How to plot a greek letter on a 3D plot label with hist3D()?

I am plotting a 3D histogram using hist3D(), and want to change xlab so that it includes a Greek letter with a subscript.

I used

xlab=expression(theta[1]). 

It doesn't work, and I just get the string "theta[1]" in the label. It works fine, on the other hand, for just the regular plot command.

So how can I introduce subscripted Greek letters to hist3D() plots?

(hist3D() is in the plot3D library)