Android ProguardRelease FAILED due to empty library of output boxes

I imported a project from Eclipse to Android Studio 1.0.1, which consists of an Android library project (RuletaAfortunadaCore) and an Android project (RuletaAfortunada), also has some third party dependencies. Everything seemed fine during import, but now when I try to build it, I get this error message from Gradle:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':ruletaAfortunadaCore:proguardRelease'.
> java.io.IOException: The output jar is empty. Did you specify the proper '-keep' options?

      

        
        
        
      

    

The whole result from the beginning of the build process:

Executing tasks: [clean, :ruletaAfortunada:compileDebugSources, :facebookSDK:compileDebugSources, :ruletaAfortunadaCore:compileDebugSources]

Configuration on demand is an incubating feature.
:facebookSDK:clean
:ruletaAfortunada:clean UP-TO-DATE
:ruletaAfortunadaCore:clean
:facebookSDK:compileLint
:facebookSDK:copyReleaseLint UP-TO-DATE
:facebookSDK:mergeReleaseProguardFiles UP-TO-DATE
:facebookSDK:preBuild
:facebookSDK:preReleaseBuild
:facebookSDK:checkReleaseManifest
:facebookSDK:prepareReleaseDependencies
:facebookSDK:compileReleaseAidl
:facebookSDK:compileReleaseRenderscript
:facebookSDK:generateReleaseBuildConfig
:facebookSDK:generateReleaseAssets UP-TO-DATE
:facebookSDK:mergeReleaseAssets
:facebookSDK:generateReleaseResValues UP-TO-DATE
:facebookSDK:generateReleaseResources
:facebookSDK:packageReleaseResources
:facebookSDK:processReleaseManifest
:facebookSDK:processReleaseResources
:facebookSDK:generateReleaseSources
:facebookSDK:compileReleaseJava
:facebookSDK:processReleaseJavaRes UP-TO-DATE
:facebookSDK:packageReleaseJar
:facebookSDK:compileReleaseNdk
:facebookSDK:packageReleaseJniLibs UP-TO-DATE
:facebookSDK:packageReleaseLocalJar
:facebookSDK:packageReleaseRenderscript UP-TO-DATE
:facebookSDK:bundleRelease
:ruletaAfortunada:preBuild
:ruletaAfortunada:preDebugBuild
:ruletaAfortunada:checkDebugManifest
:ruletaAfortunada:preReleaseBuild
:ruletaAfortunadaCore:compileLint
:ruletaAfortunadaCore:copyReleaseLint UP-TO-DATE
:ruletaAfortunadaCore:preBuild
:ruletaAfortunadaCore:preReleaseBuild
:ruletaAfortunadaCore:checkReleaseManifest
:ruletaAfortunadaCore:preDebugBuild
:ruletaAfortunadaCore:preDebugTestBuild
:ruletaAfortunadaCore:prepareComAndroidSupportSupportV42100Library
:ruletaAfortunadaCore:prepareComGoogleAndroidGmsPlayServices6587Library
:ruletaAfortunadaCore:prepareRuletaAfortunadaFacebookSDKUnspecifiedLibrary
:ruletaAfortunadaCore:prepareReleaseDependencies
:ruletaAfortunadaCore:compileReleaseAidl
:ruletaAfortunadaCore:compileReleaseRenderscript
:ruletaAfortunadaCore:generateReleaseBuildConfig
:ruletaAfortunadaCore:generateReleaseAssets UP-TO-DATE
:ruletaAfortunadaCore:mergeReleaseAssets
:ruletaAfortunadaCore:generateReleaseResValues UP-TO-DATE
:ruletaAfortunadaCore:generateReleaseResources
:ruletaAfortunadaCore:mergeReleaseResources
:ruletaAfortunadaCore:processReleaseManifest
:ruletaAfortunadaCore:processReleaseResources
:ruletaAfortunadaCore:generateReleaseSources
:ruletaAfortunadaCore:compileReleaseJava
:ruletaAfortunadaCore:extractReleaseAnnotations
:ruletaAfortunadaCore:mergeReleaseProguardFiles UP-TO-DATE
:ruletaAfortunadaCore:compileReleaseNdk
:ruletaAfortunadaCore:packageReleaseJniLibs UP-TO-DATE
:ruletaAfortunadaCore:packageReleaseRenderscript UP-TO-DATE
:ruletaAfortunadaCore:packageReleaseResources
:ruletaAfortunadaCore:proguardRelease FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':ruletaAfortunadaCore:proguardRelease'.
> java.io.IOException: The output jar is empty. Did you specify the proper '-keep' options?

      

        
        
        
      

    

Content of build.gradle file for such library project:

apply plugin: 'com.android.library'

android {
    compileSdkVersion 13
    buildToolsVersion "21.1.2"

    defaultConfig {
        minSdkVersion 9
        targetSdkVersion 17
    }

    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-project.txt'
        }
    }
}

dependencies {
    compile project(':facebookSDK')
    compile 'com.android.support:support-v4:18.0.0'
    compile 'com.google.code.gson:gson:2.2.4'
    compile 'com.google.android.gms:play-services:+'
    compile files('libs/chartboost.jar')
    compile files('libs/mint-4.0.7.jar')
}

      

        
        
        
      

    

UPDATE . I figured out that an Android library project called RuletaAfortunadaCore can be created as a standalone project or built as a module with no problem. It only fails to build when building the whole RuletaAfortunada Android project that uses it.

The build.gradle file for such an Android project is the default one generated by Android Studio, I guess so there is nothing interesting inside. Anyway, here it is, just in case:

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.0.0'
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

      

        
        
        
      

    

These are the .gradle settings:

include ':facebookSDK'
include ':ruletaAfortunadaCore'
include ':ruletaAfortunada'

      

        
        
        
      

    

And finally, for all of them to be, this is the facebookSDK build.gradle module:

apply plugin: 'com.android.library'

android {
    compileSdkVersion 9
    buildToolsVersion "21.1.2"

    defaultConfig {
        minSdkVersion 9
        targetSdkVersion 9
    }

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

dependencies {
    compile 'com.android.support:support-v4:18.0.0'
    compile files('libs/bolts-android-1.1.2.jar')
}