How to get a string field value via reflection?

I have this method that returns a list of Fields objects that are of String type.

    public static List<Field> getStringFields(Class<?> clazz) {
        List<Field> toReturn = new ArrayList<Field>();

        Field[] allFields = clazz.getDeclaredFields();

        for (Field f : allFields) {
            Class<?> type = f.getType();
            if (type == String.class) {
                stringFields.add(f);
            }
        }

        return stringFields;
    }

And this method, that should captalize all the string fields:

public void capitalizeStringFields() {
    List<Field> stringFieldsToCapitalize = Utils.getStringFields(someClass.class);
    try {
        for (Field field : stringFieldsToCapitalize) {
            field.set(this, field.get(this).toString().toUpperCase());
        }
    } catch (IllegalArgumentException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

The problem is: the "field.set(...)" is not working. It supposed to capitalize the value and set it as the new value for that field, but it's not working... Any idea of how can I solve this? (PS: In the real code the "someClass.class" is set as the real class name...)

The error being generated is IllegalArgumentException in the field.get(this) method (insde the field.set method).