In CronExpression.storeExpressionVals, it tries to calculate month number in the follow snippet:


            if (type == MONTH) {
                sval = getMonthNumber(sub) + 1;
                if (sval < 0) {
                    throw new ParseException("Invalid Month value: '" + sub + "'", i);
                }
                if (s.length() > i + 3) {
                    c = s.charAt(i + 3);
                    if (c == '-') {
                        i += 4;
                        sub = s.substring(i, i + 3);
                        eval = getMonthNumber(sub) + 1;
                        if (eval < 0) {
                            throw new ParseException("Invalid Month value: '" + sub + "'", i);
                        }
                    }
                }
            }


The method getMonthNumber returns -1 for invalid values.
1 is added to the returned value and then a check for less than zero is done. This will never be less than zero.

This is not critical as it fails later with the exception: "Month values must be between 1 and 12"
Though the other message is more appropriate.

Prior to splitting out of the CronExpression, this fails in CronTrigger.java