Problem initializing array of union

gistfile1.rb

# It seems this is the wrong way to create an array of union in FFI. What's the right way?

array = []

# FileSegmentElement is an FFI::Union, and :Alignment is a :uint64
for i in 0...page_num
  segment = FileSegmentElement.new
  segment[:Alignment] = base_address + page_size * i
  array << segment
end

# Add an extra element for null
array << FileSegmentElement.new

segment_array = FFI::MemoryPointer.new(FileSegmentElement, array.length)
segment_array.put_array_of_pointer(0, array)

# Seems to work but a call to ReadFileScatter simply returns "the parameter is incorrect".