Also do this:

   $stderr.puts foo.class

Is it possible that one of your arguments is a BigDecimal?

$ irb --simple-prompt
>> require 'bigdecimal'
=> true
>> foo = BigDecimal.new("0.713")
=> #<BigDecimal:7fc0698c7180,'0.713E0',9(18)>
>> foo ** 0.5555555
TypeError: wrong argument type Float (expected Fixnum)
  from (irb):3:in `**'
  from (irb):3
  from :0

(this will be fixed in 1.9.3, according to NEWS file)

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