mergefonts and OTFs

I need help with mergefonts.
I am trying to exchange the k in src.otf for the k from k.otf. This is the command line I came up with:
mergefonts -gx k output.otf src.otf glyphalias.txt k.otf

The glyphalias.txt file reads as follows:
mergeFonts k k

I receive the following output:
mergefonts: --- src.otf mergefonts: (cfr) /BlueScale too large/small (ignored) mergefonts: --- k.otf mergefonts: (cfr) /BlueScale too large/small (ignored)

And a new file called output.otf is indeed generated, but it is not a valid OTF. It doesn’t start with the OTTO header. I posted a screenshot of the file opened in TextMate: http://cl.ly/image/3X0e2J1j0p1P

Can someone enlighten me:
1. How do I get mergefonts to output OTFs?
2. And what file format is it in now?
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