JNI with multidimensional Byte Array

Hi all ,

     I am a newbie in JNI . I need to pass a two dimensional byte array
through JNI .

The snippet of JNI function is like this :

JNIEXPORT jstring JNICALL Java_com_test_invokeTest
  (JNIEnv *env, jobject cl, jobjectArray detail)
{
    int i ,nRows ;
     jobject col;
    char *Details[20];

   nRows = (*env)->GetArrayLength(env, detail);
    for (i = 0; i < nRows; i++)
    {
     col = (*env)->GetObjectArrayElement(env, detail, i);
     Details[i] = (*env)->GetCharArrayElements(env, col, NULL);
        printf("Details[%d] : %s\n",i,Details[i]);
   }

     for(i = 0; i < nRows; i++)
        (*env)->ReleaseCharArrayElements(env, col, Details[i],NULL);

    return ((*env)-> NewStringUTF(env,str));

}

When I am passing a 2-dimensional byte array and executing it without
any java flags , it works properly most of the time (occasionally it
was crashing the JVM ) . So I tried using flags "-Xcheck:jni -Xfuture"
as suggested by Sun . When I used this flags , it is giving me this
error

FATAL ERROR in native method: Array element type mismatch in JNI.

It's crashing just before the call Details[i] =
(*env)->GetCharArrayElements(env, col, NULL);

Can anyone tell me what's wrong with this call .. or should I use some
other call ?

Thanks a lot of your help !