OpenCV Error: Null pointer (Hidden cascade has not been created. Use cvSetImagesForHaarClassifierCascade)

[yocontra]

OpenCV Error: Null pointer (Hidden cascade has not been created.
Use cvSetImagesForHaarClassifierCascade) in cvRunHaarClassifierCascadeSum, file /tmp/opencv-pKns/OpenCV-2.4.2/modules/objdetect/src/haar.cpp, line 671
libc++abi.dylib: terminate called throwing an exception
Abort trap: 6

I get this when running

cv = require 'opencv'

buf = (some png buffer)

cv.readImage buf, (err, im) ->
      return cb err if err?
      im.detectObject "./haarcascade_frontalface_alt.xml", {}, (err, faces) ->
        return cb err if err?
        return cb null, faces