Reusing function variables in a `when` conditional

I have a function with a condition:

  def iteraterefs(divisor, number, table, out, i) when number >= divisor do
    number = number - divisor
    out = out <> elem(table, i)
  end

However it seems that the compiler does not like the condition when number >= divisor do, is there something about reuse of function variables I am missing from the documentation?