Apps Bug with Proguard minifyEnabled on A/S 1.5.1 ?

[ac4android]

This might be a bug, don't know whether it has been reported.

I tried to compile an apk-release.apk using "Build -> Generate signed APK..." and A/S 1.5.1 threw this error:
Error:Execution failed for task ':app:transformClassesAndResourcesWithProguardForRelease'. > java.io.IOException: Please correct the above warnings first

I went into build.gradle and disabled ninifyEnabled, like this :

buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}

Then it worked well.

QUESTION: I'd rather have proguard minifyEnabled to scramble the code, is there a work around?