If you don't specify a Comparator, it uses "natural order" (for anything that sorts. You could fix this by instantiating the TreeMap with the following Comparator:

public class IgnoreCaseStringComparator implements Comparator {

public int compare(Object o1, Object o2) {

((String)o1).compareToIgnoreCase((String)o2);
}
}

also check if they are null in the comparator, and if they are both String objects.

(I love Comparators)