RMagick question

I'm trying to convert several (300) .tif files to .gif files using
RMagick.

But I'm getting the following message:

  irb(main):030:0> i = Image::read("ir_mmw_000.tif").first
  => ir_mmw_000.tif TIFF 640x480 PseudoClass 256c 8-bit 311238b
  irb(main):031:0> i.class
  => Magick::Image
  irb(main):032:0> i.write "foo.gif"
  Magick::ImageMagickError: pixel cache is not open `foo.gif':
        from (irb):32:in `write'
        from (irb):32

Any ideas?

Phil

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