NullPointerException when unboxing Integer?

This code is causing a null pointer exception. I have no idea why:

private void setSiblings(PhylogenyTree node, Color color) throws InvalidCellNumberException {
    PhylogenyTree parent = node.getParent();

    for (PhylogenyTree sibling : parent.getChildren()) {
        if (! sibling.equals(node)) {
            Animal animal = sibling.getAnimal();
            BiMap<PhylogenyTree, Integer> inverse = cellInfo.inverse();
            int cell = inverse.get(animal); // null pointer exception here
            setCellColor(cell, color);
        }
    }
}

I've examined it in the debugger, and all the local variables are non-null. How else could this be happening? The BiMap is from Google Collections.