Error in parsing. For some cases number is null

[rabestro]

            if (s.indexOf('+') > 0) {
                re = s.substring(0, s.indexOf('+'));
                im = s.substring(s.indexOf('+') + 1);
                parsed = new ComplexNumber(Double.parseDouble(re), Double.parseDouble(im));
            } else if (s.lastIndexOf('-') > 0) {
                re = s.substring(0, s.lastIndexOf('-'));
                im = s.substring(s.lastIndexOf('-') + 1);
                parsed = new ComplexNumber(Double.parseDouble(re), -Double.parseDouble(im));
            } else {
                parsed = null;
            }