Negative TimeZoneOffset newIncorrectDataException

When trying to retrieve a negative timezone, I get an exception quickfix.FieldMap.newIncorrectDataException(FieldMap.java:410).

It seems that it is caused by the following code in IntConverter.java, it expects only digits.
public static int convert(String value) throws FieldConvertError {
try {
for (int i = 0; i < value.length(); i++) {
if (!Character.isDigit(value.charAt))

{ throw new NumberFormatException(); }

return Integer.parseInt(value);
...

The code I am using is as follows:

TimezoneOffset timezoneOffset = new TimezoneOffset();
try {
noMDEntries.get(timezoneOffset);
} catch(FieldNotFound e) {
//field is optional
}