android - デバッグ ビルドでの Android Proguard エラー

Question

Debug ビルドでの Proguard の動作を見たかったのです。Android Studio を使用した私の build.gradle を次に示します。

buildTypes {
        debug {
            minifyEnabled true
            proguardFile getDefaultProguardFile('proguard-android-optimize.txt')
        }
    }

ビルドが失敗し、次の出力が得られます。

:proguardDebug
Warning: JavaAPI.CavvPreAuth: can't find referenced class sun.misc.BASE64Decoder
Warning: JavaAPI.CavvPreAuth: can't find referenced class sun.misc.BASE64Decoder
Warning: JavaAPI.CavvPreAuth: can't find referenced class sun.misc.BASE64Decoder
Warning: JavaAPI.CavvPurchase: can't find referenced class sun.misc.BASE64Decoder
Warning: JavaAPI.CavvPurchase: can't find referenced class sun.misc.BASE64Decoder
Warning: JavaAPI.CavvPurchase: can't find referenced class sun.misc.BASE64Decoder
Warning: there were 6 unresolved references to classes or interfaces.
         You may need to add missing library jars or update their versions.
         If your code works fine without the missing classes, you can suppress
         the warnings with '-dontwarn' options.
         (http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedclass)
:proguardDebug FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':proguardDebug'.
> java.io.IOException: Please correct the above warnings first.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

何か案は？