How to initialise a three dimensional ruby array

I want to initialise a three dimensional ruby array. For a two-dimensional array, I can do

a = Array.new(4){ Array.new(5, 0) }

so I have tried

a = Array.new(4) { Array.new(5, Array.new(6, 0)) }

but if I do a[1][2][3] = 5, it not only sets that element to 5, it sets the corresponding element to 5 in other sub arrays i.e.

[[[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]],
 [[0, 0, 0, 5, 0, 0], [0, 0, 0, 5, 0, 0], [0, 0, 0, 5, 0, 0], [0, 0, 0, 5, 0, 0], [0, 0, 0, 5, 0, 0]],
 [[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]],
 [[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]]]