SQLite return -1 in android

i am working with SQLite in android and i am facing a problem. can anybody tell me that following method return -1 why ?

     public long addNamazTime(Namaz namaz)
     {
            SQLiteDatabase db = this.getWritableDatabase();
            ContentValues values = new ContentValues();
            Log.d("Tag", "Date: "+namaz.getDate());
            values.put(KEY_DATE, namaz.getDate());

            Log.d("Tag", "Fajar"+ namaz.getFajar());
            values.put(KEY_FAJAR, namaz.getFajar());

            Log.d("Tag", "Zohr: "+namaz.getDhuhr());
            values.put(KEY_DHUHR, namaz.getDhuhr());

            Log.d("Tag", "Asar: "+namaz.getAsar());
            values.put(KEY_ASAR, namaz.getAsar());

            Log.d("Tag", "Maghrib: "+namaz.getMaghrib());
            values.put(KEY_MAGHRIB, namaz.getMaghrib());

            Log.d("Tag", "Isha: "+namaz.getIsha());
            values.put(KEY_ISHA, namaz.getIsha());

            // Inserting Row
            long id = db.insert(TABLE_NAME, null, values);
            return id;
        }