Gradle et proguard: impossible de trouver la méthode runProguard() pour les arguments [vrai]

J'ai configuré le construire.gradle comme suggéré par Proguard Gradle manuel

C'est la racine de construire.gradle

buildscript {
    repositories {
        flatDir dirs: '/home/username/android-sdks/tools/proguard/lib'
        mavenCentral()
    }
    dependencies {                     
        classpath 'com.android.tools.build:gradle:0.5.+'
        classpath ':proguard'
    }
}

Maintenant, c'est la construire.gradle pour mon projet

apply plugin: 'android'

dependencies {
    compile fileTree(dir: 'libs', include: '*.jar')
    compile project(':SomeLibraryProject')
}

android {
    compileSdkVersion 19
    buildToolsVersion "19.0.0"

    sourceSets {
        ...
    }

    task runProguardTask(type: proguard.gradle.ProGuardTask) {
    }

    signingConfigs {
        debug {
            storeFile file("./keystore/keystore")
            storePassword "******"
            keyAlias "******"
            keyPassword "*******"
        }

        release {
            runProguard true
            proguardFile 'proguard-android.txt'
            storeFile file("./releasekey/keystore")
            storePassword "******"
            keyAlias "********"
            keyPassword "*******"
        }
    }

    buildTypes {
        release {
            signingConfig signingConfigs.release
        }
    }  
}

Et c'est la sortie

$ ./gradlew build

FAILURE: Build failed with an exception.

* Where:
Build file '/home/username/Documents/eclipse/workspace/repo/ProjectName/build.gradle' line: 49

* What went wrong:
A problem occurred evaluating project ':ProjectName'.
> Could not find method runProguard() for arguments [true] on SigningConfigDsl_Decorated{name=release, storeFile=null, storePassword=null, keyAlias=null, keyPassword=null, storeType=null}.

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

BUILD FAILED

Total time: 9.14 secs

Je me demande aussi pourquoi le type storefile, storePassword, keyAlias et keyPassword sont nuls?