Fields in java, "not a primitive field" error

I have been working with Fields for parsing a class to a SharedPreferences. It worked while the settings class was not a Singleton. I changed to work as a Singleton and it is not working anymore.

G2ASettings settings = G2ASettings.getInstance();
    Field[] fields = G2ASettings.class.getDeclaredFields();
    for (Field f : fields) {
        f.setAccessible(true);
        try {
            if (preferences.contains(f.getName()) && !f.getName().equals("INSTANCE")) {
                f.set(settings, preferences.getBoolean(f.getName(), f.getBoolean(settings))); //si no lo encuentra, pone el valor por defecto determinado en la clase
            } else {
                if (blablabla) {
                    editor.putBoolean(f.getName(), true);
                    allPreferencesDisabled = true;
                } else
   (*)----->        editor.putBoolean(f.getName(), f.getBoolean(settings)); 
            }
   (*)----->  if (!allPreferencesDisabled) allPreferencesDisabled = f.getBoolean(settings);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

The error reported is the following:

Caused by: java.lang.IllegalArgumentException: not a primitive field

The error is repoted at lines marked as (*) in the code.

The fields of G2ASettings are all public and boolean except INSTANCE variable which is G2ASettings type variable and is private.

I will be reporting here my progress.

EXTRA: I post here only relevant data of G2ASettings class (usual Singleton class):

public class G2ASettings implements Serializable {

public boolean {big amount of boolean variables]

private static G2ASettings INSTANCE = null;
private synchronized static void createInstance() {
    if (INSTANCE == null) {
        INSTANCE = new G2ASettings();
    }
}

public static G2ASettings getInstance() {
    if (INSTANCE == null) createInstance();
    return INSTANCE;
}

{public getters and setters}
}