2.3手机上用backdrawble,切换时背景无法改变，求助！

<com.ftd2009.switchbuttoniosdemo.SwitchButton
    android:id="@+id/sb_ios"
    style="@style/SwitchButtonStyle"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:kswAnimationDuration="300"
    app:kswBackDrawable="@drawable/ios_back_drawable"
    app:kswBackMeasureRatio="2"
    app:kswThumbDrawable="@drawable/ios_thumb_selector"/>

用的是app:kswBackDrawable，@drawable/ios_back_drawable是：

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="false" android:state_checked="true">
        <shape>
            <solid android:color="#3097fd"/>
            <corners android:radius="15dp"/>
        </shape>
    </item>
    <item android:state_enabled="false" android:state_checked="false">
        <shape>
            <solid android:color="#ff0000"/>
            <corners android:radius="25dp"/>
        </shape>
    </item>
    <item android:state_checked="true">
        <shape>
            <solid android:color="#000000"/>
            <corners android:radius="35dp"/>
        </shape>
    </item>
    <item>
        <shape>
            <solid android:color="#ffff00"/>
            <corners android:radius="45dp"/>
        </shape>
    </item>
</selector>

但是在2.3上切换时改变不了背景色，4.0上都是好的，是什么问题啊。我用的nineold库可以在2.3上有动画，但是切换时背景色改变不了，HELP！
看了看源码，应该是下面这块代码有问题：

if (!mIsBackUseDrawable && mBackColor != null) {
            mCurrBackColor = mBackColor.getColorForState(getDrawableState(), mCurrBackColor);
            mNextBackColor = mBackColor.getColorForState(nextState, mCurrBackColor);
        } else {
            if (mBackDrawable instanceof StateListDrawable && mFadeBack) {
                mBackDrawable.setState(nextState);
                mNextBackDrawable = mBackDrawable.getCurrent().mutate();
            } else {
                mNextBackDrawable = null;
            }
            setDrawableState(mBackDrawable);
            if (mBackDrawable != null) {
                mCurrentBackDrawable = mBackDrawable.getCurrent().mutate();
            }
        } 

Start with the SwitchButton state-handling code shown in the issue, especially the mBackDrawable and setDrawableState branches. Reproduce the drawable background transition on Android 2.3 and 4.0 using the supplied XML, then compare the resulting drawable state. Done means the background color changes correctly when the switch changes state on both versions.