Android compatibility ExceptionInInitializerError

I'm using a Compatibility class to build a User Agent string:

public abstract class Compatibility {

    private static int sdkInt = 0;

    private static Compatibility instance;

    public static Compatibility getInstance(){
        if (instance!=null) return instance;

        try { sdkInt = Integer.parseInt(Build.VERSION.SDK); } catch (Exception e) {
            sdkInt = 3;
        }
        if (sdkInt<4) {
            instance = new CompatibilityAPI3();
        } else {
            instance = new CompatibilityAPI4();
        }
        return instance;
    }

    public int getSdkInt(){
        return sdkInt;
    }

    public abstract String getUa();

}


public class CompatibilityAPI3 extends Compatibility {
    @Override
    public String getUa() {
        return "unknown|unknown|"+Build.DEVICE+"|"+Build.PRODUCT+"|"+Build.BRAND+"|"+Build.VERSION.SDK;     
    }
}


public class CompatibilityAPI4 extends Compatibility {
    @Override
    public String getUa() {
        return Build.MANUFACTURER+"|"+Build.MODEL+"|"+Build.DEVICE+开发者_运维问答"|"+Build.PRODUCT+"|"+Build.BRAND+"|"+Build.VERSION.SDK;
    }
}

I see this Error happening on some devices:

java.lang.ExceptionInInitializerError
....
Caused by: java.lang.NoSuchFieldError: ?
at ...CompatibilityAPI4.getUa(CompatibilityAPI4.java:8)

What am I doing wrong?

Use reflection. See: determine API level for platforms < 1.6