Why am I getting an java.lang.IllegalArgumentException: the bind value at index 1 is null in this case?

I know what an java.lang.IllegalArgumentException: the bind value at index 1 is null means and what causes the error. I my case I am still unable to find the reason for such an error in my case.

So, what do I intend to achieve?

I have 4 columns in my SQLite database table. I want to find the sum of each column and again find the sum of these four results. The row values for sum is filtered based on the product name chosen by the user from the autoCompleteTextView.

Where am I getting the error?

my stackTrace says I am getting the error in the line beginning with Cursor c = db.query... in the below code. The following code is part of my db file of my android project.

    public int addPurchaseQuantity(String itemName) {
    SQLiteDatabase db = helper.getReadableDatabase();
    int result = 0;
    String selection = VivzHelper.COLUMN_ADD_PURCHASE_ITEM_NAME + " =? ";
    String[] selectionArgs = {itemName};
    Cursor c = db.query(VivzHelper.ADD_PURCHASE_TABLE,
            new String[]{"sum(" + VivzHelper.COLUMN_ADD_PURCHASE_ITEM_QUANTITY + ")"},
            selection,
            selectionArgs,
            null,
            null,
            null);
    if (c.moveToFirst()) {
        result = c.getInt(0);
    }
    c.close();
    return result;
}

The itemName is obtained from the autoCompleteTextView based on user selection.

        String[] autoCompleteName = vivzHelper.getInventoryNameFilterBySupplierName(vivzHelper.getSupplierID(param1));
    ArrayAdapter<String> NameAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, autoCompleteName);
    addPurchaseItemName.setThreshold(1);// starts working from first char
    addPurchaseItemName.setAdapter(NameAdapter);

    addPurchaseItemName.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, long arg3) {
            itemName = String.valueOf(arg0.getItemAtPosition(arg2));
        }
    });

My stackTrace is also pointing to the following code of the java activity file.

int purchaseQty = vivzHelper.addPurchaseQuantity(itemName);

Here is my stackTrace

04-21 00:09:58.274  21975-21975/com.example.bharathduraiswamy.comboedittext E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.bharathduraiswamy.comboedittext/com.example.bharathduraiswamy.comboedittext.AddPurchase}: java.lang.IllegalArgumentException: the bind value at index 1 is null
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2313)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2365)
        at android.app.ActivityThread.access$600(ActivityThread.java:156)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:153)
        at android.app.ActivityThread.main(ActivityThread.java:5336)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.IllegalArgumentException: the bind value at index 1 is null
        at android.database.sqlite.SQLiteProgram.bindString(SQLiteProgram.java:164)
        at android.database.sqlite.SQLiteProgram.bindAllArgsAsStrings(SQLiteProgram.java:200)
        at android.database.sqlite.SQLiteDirectCursorDriver.query(SQLiteDirectCursorDriver.java:47)
        at android.database.sqlite.SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase.java:1314)
        at android.database.sqlite.SQLiteDatabase.queryWithFactory(SQLiteDatabase.java:1161)
        at android.database.sqlite.SQLiteDatabase.query(SQLiteDatabase.java:1032)
        at android.database.sqlite.SQLiteDatabase.query(SQLiteDatabase.java:1200)
        at com.example.bharathduraiswamy.comboedittext.VivzDatabaseAdapter.addPurchaseQuantity(VivzDatabaseAdapter.java:2011)
        at com.example.bharathduraiswamy.comboedittext.AddPurchase.aggregateQty(AddPurchase.java:531)
        at com.example.bharathduraiswamy.comboedittext.AddPurchase.onCreate(AddPurchase.java:133)
        at android.app.Activity.performCreate(Activity.java:5122)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2277)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2365)
        at android.app.ActivityThread.access$600(ActivityThread.java:156)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:153)
        at android.app.ActivityThread.main(ActivityThread.java:5336)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
        at dalvik.system.NativeStart.main(Native Method)

Can someone tell me where actually am I going wrong?