Android : (*env)->GetArrayLength(env, bytes) always returns 0

I'm new to Android Development, and just want to ask about little JNI code, I'm having a problem getting the right length of array passed in this code.

Here is my JNI code:

// jniexample.c
JNIEXPORT int JNICALL Java_com_example_HelloAndroid_getLength(JNIEnv * env, jobject obj, jbyteArray bytes)
{
     jsize len = (*env)->GetArrayLength(env, bytes);
     return len;
}

and here is the java code that calls it:

// com.example.HelloAndroid
public class HelloAndroid extends Activity
{
     static
     {
          System.loadLibrary("jniexample");
     }

     public native int getLength(byte[] bytes);

     @Override
     protected void onCreate(Bundle savedInstanceState)
     {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.chilli_main);
          byte[] b = new byte[] { 0, 0, 0, 0, 0, 0, 0 };
          new AlertDialog.Builder(this).setMessage(
            "" + getLength(b)).show();
     }
}

Everything works properly except that the method getLength(byte[]) always return 0.

Here is the link for screenshot (since I cannot post images yet)

http://i.imgur.com/NktDobH.jpg

Thanks in advance.