png-js: cannot read property '8' of undefined

Whenever I try to use png-js in Node.js, I get this strange error:

/home/anderson/node_modules/png-js/png-node.js:152
      b1 = this.data[this.pos++] << 24;
                    ^
TypeError: Cannot read property '8' of undefined
    at PNG.readUInt32 (/home/anderson/node_modules/png-js/png-node.js:152:21)
    at new PNG (/home/anderson/node_modules/png-js/png-node.js:56:26)
    at /home/anderson/node_modules/png-js/png-node.js:34:15
    at [object Object].<anonymous> (fs.js:88:5)
    at [object Object].emit (events.js:67:17)
    at Object.oncomplete (fs.js:1059:12)

Here's the program that produced this output:

var PNG = require('png-js');
PNG.decode('some.png', function(pixels) {
    // pixels is a 1d array of decoded pixel data
});

In the past, I've been able to use the png-js module without any trouble. Is it possible that this module might conflict with another node.js module that I've installed?