Username/Password validation SQLite Android

Question

I am trying to make a login page in Android, but the program crashes every time I run it. Here's my code:

b1.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {
            // TODO Auto-generated method stub
            String a = ed1.getText().toString();
            String b = ed2.getText().toString();
            String repassword;
            if (a.length() <= 0 || b.length() <= 0) {
                if (a.length() <= 0) {
                    ed1.setError("Please enter your username.");
                }
                if (b.length() <= 0) {
                    ed2.setError("Please enter your password.");
                }
            } else {
                Cursor cursor = sq
                        .query("AccountRegistration", null, " Username=?",
                                new String[] { a }, null, null, null);

                if (cursor.getCount() < 1) {
                    cursor.close();
                    repassword = "NOT EXIST";
                }
                cursor.moveToFirst();

                repassword = cursor.getString(cursor
                        .getColumnIndex("Password"));
                cursor.close();

                String storedPassword = repassword;

                if (b.equals(storedPassword)) {
                    Toast.makeText(getApplicationContext(),
                            "You have been logged in.", Toast.LENGTH_LONG)
                            .show();
                    Intent ii = new Intent(MainActivity.this,
                            WelcomePage.class);
                    startActivity(ii);
                } else {
                    Toast.makeText(
                            getApplicationContext(),
                            "Please check your credentials or create a new account.",
                            Toast.LENGTH_LONG).show();
                    ed1.setText("");
                    ed2.setText("");
                }
            }
        }
    });

When I tried debugging, I noticed the error was at the part where I store cursor.getString(cursor.getColumnIndex("Password")) into the repassword variable. My logcat says cursorindexoutofbounds.