How to convert 32 bytes char buffer into uint64 in lua?

Question

I am writing a mechanism to generate a unique ID derived from sha256. I use:

require 'sha2'

function getUint64ID( callId )
  local minimumValue = 100000000000000000
  local maximumValue = 999999999999999999

  outputHash = sha2.sha256hex(callId)
  print(outputHash .. "\n")

  local c1 = ""

  for a, b in outputHash:gmatch"(%x)(%x)" do
    hexTuple = tostring(a) .. tostring(b)
    intVal   = tonumber(hexTuple, 16)
    c1 = c1 .. string.char(intVal)
  end

  uint64ID = ( minimumValue + ( tonumber(c1) % ( maximumValue - minimumValue + 1 ) ) )

  print('uint64ID:')
  print(string.format("%18.0f",uint64ID))
end

getUint64ID("test")

I get the following error in the above code:

stdin:17: attempt to perform arithmetic on a nil value
stack traceback:
    stdin:17: in function 'getUint64ID'
    stdin:1: in main chunk
    [C]: ?

How to convert 32 bytes char buffer (c1) into a uint64 number in lua?