gradle-play-publisher: Could not find ZipAlign task with valid signingConfig

gradle-play-publisher fails although I specified a valid signingConfig, looking like this:

signingConfigs {
        debug {
            storeFile file("../keystore.jks")
            storePassword keystorePassword // coming from local.properties
            keyAlias "alias"
            keyPassword keystorePassword
        }
        release {
            storeFile file("../keystore.jks")
            storePassword keystorePassword
            keyAlias "alias"
            keyPassword keystorePassword
        }
    }
// ...
buildTypes {
        release {
            signingConfig signingConfigs.release
            minifyEnabled true
            shrinkResources true
            zipAlignEnabled true // Tested with and without this parameter.

            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }

        debug {
            signingConfig signingConfigs.debug
            applicationIdSuffix '.debug'
        }
    }

Error:

Could not find ZipAlign task. Did you specify a signingConfig for the variation InternalRelease?
Could not find ZipAlign task. Did you specify a signingConfig for the variation ProductionRelease?

I am using the Android Plugin version 2.2.0-alpha1.