D3 sort function always passes undefined arguments

Using D3 2.4.2, I create a number of path elements like so:

for (var i = 0; i < pathIndices.length; i++) {
  graph.append("svg:path")
    .style("stroke", colors[pathIndices[i]])
    .style("stroke-width", "2.5px")
    .style("fill", "none")
    .attr("class", PATH_CLASS)
    .attr("id", PATH_ID_PREFIX + pathIndices[i])
    .attr("d", lineFunc(data))[0];
}

They all draw to the screen as expected. Later on, I want to bring one of them to the front when the user makes some input, so I have an event handler that does this:

var pathToHighlight = selectPath(pathIndex);
var paths = d3.selectAll("." + PATH_CLASS);
paths.sort(
  function(a, b) {
    if (a === pathToHighlight) {
      return -1;
    }
    else if (b === pathToHighlight) {
      return 1;
    }
    else {
      return 0;
    }
  }
);

Setting breakpoints in Chrome indicates that my path selections here are successful (paths is an array of SVGPathElements). But the code does nothing, and setting breakpoints inside the sort function shows that a and b are always undefined. Going up into the d3 code, I see that when the internal function d3_selection_sortComparator calls my comparator with the appropriate arguments, except they're ANDed with their own undefined __data__ members, which causes undefined to be passed in:

// a and b are correct, but a.__data__ and b.__data__ are undefined
return comparator(a && a.__data__, b && b.__data__);

What am I doing wrong here? My paths draw correctly to the screen, so it seems like they should have the correct data. Right?

Edit: Images: