Android: Why is the RadioButton returning a NULL value?

I created a RadioGroup to enable the gender selection.

Here's the part of the XML file that declares the RadioGroup and the RadioButtons.

        <RadioGroup
            android:id="@+id/rgGenderGroup"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal">

            <RadioButton
                android:id="@+id/rbMale"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="0.5"
                android:text="@string/m"
                android:textColor="@color/default_text"/>

            <RadioButton
                android:id="@+id/rbFemale"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="0.5"
                android:text="@string/f"
                android:textColor="@color/default_text"/>

        </RadioGroup>

And now, in the activity, I declared the RadioGroup and RadioButton variables and overrode the onCheckedChanged method. To return a certain value of each choice, I also declared a String value.

private String gender = "";
private RadioGroup rgGenderGroup;
private RadioButton rbMale, rbFemale;

...

    @Override
    public void onCheckedChanged(RadioGroup group, int checkedId) {
        switch(checkedId) {
            case(R.id.rbMale):
                gender = "M";
                break;
            case(R.id.rbFemale):
                gender = "F";
                break;
        }
    }

But, as I tried to insert the gender value into the database with the query below,

private final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + " ( " +
        COLUMN_NAME + " TEXT NOT NULL, " + COLUMN_AGE + " INT, " + COLUMN_NATIONALITY + " TEXT NOT NULL, " +
        COLUMN_GENDER + " CHAR(1) NOT NULL, " + COLUMN_CREATED_AT + " TEXT NOT NULL);";

It fails, showing the following error message.

08-10 23:46:52.356  21666-21890/com.marshall.sortingtester E/SQLiteDatabase﹕ Error inserting Name=clara Age=0 Nationality=Angola Created_At=2015/08/10 23:46:52 Gender=null
    android.database.sqlite.SQLiteConstraintException: People.Gender may not be NULL (code 19)

Why is the gender value returning null even after checking the button properly?

EDIT: Here's the insertData method.

public void insertData(Person person) {
        ContentValues values = new ContentValues();
        values.put(SQLiteHandler.COLUMN_NAME, person.getName());
        values.put(SQLiteHandler.COLUMN_AGE, person.getAge());
        values.put(SQLiteHandler.COLUMN_NATIONALITY, person.getNationality());
        values.put(SQLiteHandler.COLUMN_GENDER, person.getGender());
        values.put(SQLiteHandler.COLUMN_CREATED_AT, person.getCreated_at());
        db.insert(SQLiteHandler.TABLE_NAME, null, values);
    }