I'd like to use a selector to grey out an icon when it is not enabled. It seems that something like this should work (with a white background):

```
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="false" >
      android:alpha="0.5"
    </item>

    <item android:state_enabled="true" >
      android:alpha="1"
    </item>
</selector>

```

But it results in a java.lang.NumberFormatException during run time. I also tried "0.5f". Same error.

This is similar to [Is there a way to set drawable's Alpha using XML?](https://stackoverflow.com/questions/8179250/is-there-a-way-to-set-drawables-alpha-using-xml) but I'm specifically asking about the NumberFormatException. Incidentally, I also tried using integer values between 0 and 255. I get the same error.