Wrong number of arguments when calling write_array_of_type

[nikolas-n]

Hello all, I'd like to ask your help. If you believe that this shouldn't be here, feel free to close it.
I'm trying to run sippy_cup and specifically this part of the code

# ffi interface to freeswitch's g711
require 'ffi'

module SippyCup
  module G711
    extend FFI::Library
    ffi_lib 'spandsp'

    enum :EncodeState, [
      :G711_ALAW, 0,
      :G711_ULAW, 1,
    ]

    class G711State < FFI::Struct
      layout :mode,  :EncodeState
    end

    attach_function :g711_encode, [ G711State, :pointer, :pointer, :int ], :int

    def encode(samples)
      state = G711State.new
      state[:mode] = :G711_ULAW # u-law only

      iptr = FFI::MemoryPointer.new(:int16, samples.size)
      optr = FFI::MemoryPointer.new(:uint8, samples.size)

	  #puts samples.join(' ')
      iptr.write_array_of_type(:int16, :write_int16, samples)
      g711_encode(state, optr, iptr, samples.size)
      output = optr.read_array_of_type(:uint8, :read_uint8, samples.size)
	  #puts output.join(' ')
      output
    end
    module_function :encode
  end
end

And I'm getting this

Compiling media to /home/nik/github/sippy_cup/play_audio.pcap...Traceback (most recent call last):
	15: from /usr/local/bin/sippy_cup:23:in `<main>'
	14: from /usr/local/bin/sippy_cup:23:in `load'
	13: from /home/nik/.gems/gems/sippy_cup-0.7.2/bin/sippy_cup:64:in `<top (required)>'
	12: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/scenario.rb:878:in `compile!'
	11: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/scenario.rb:975:in `compile_media'
	10: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/media.rb:44:in `compile!'
	 9: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/media.rb:44:in `each'
	 8: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/media.rb:95:in `block in compile!'
	 7: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/media.rb:95:in `times'
	 6: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/media.rb:109:in `block (2 levels) in compile!'
	 5: from /home/nik/.gems/gems/sippy_cup-0.7.2/lib/sippy_cup/g711.rb:28:in `encode'
	 4: from /var/lib/gems/2.7.0/gems/ffi-1.15.5/lib/ffi/pointer.rb:134:in `write_array_of_type'
	 3: from /var/lib/gems/2.7.0/gems/ffi-1.15.5/lib/ffi/pointer.rb:134:in `each_with_index'
	 2: from /var/lib/gems/2.7.0/gems/ffi-1.15.5/lib/ffi/pointer.rb:134:in `each'
	 1: from /var/lib/gems/2.7.0/gems/ffi-1.15.5/lib/ffi/pointer.rb:136:in `block in write_array_of_type'
/var/lib/gems/2.7.0/gems/ffi-1.15.5/lib/ffi/pointer.rb:136:in `write_int16': wrong number of arguments (given 2, expected 1) (ArgumentError)

Do you know why this happens? I tried to check similar errors on the web and in the issues of this repo, but didn't find much...
Apologies if it's something you've already covered elsewhere.
Thanks!