Checkbox with custom background and no border doesn't check

I have a custom checkbox with no border and custom background:

<CheckBox
    android:id="@+id/ricordami"
    android:layout_width="25dp"
    android:layout_height="25dp"
    android:layout_gravity="center_vertical"
    android:background="@color/gray"
    android:button="@android:color/transparent" />

but when i launch my app, checkbox on click doesn't show check flag. How can i fix it?