Robolectric NoClassDefFoundError with external library

all my project activities inherits from a CustomActivity that is a external library i've created to help me with common usage things.

The problem is when i try to test that activity with robolectric i get this error:

java.lang.NoClassDefFoundError: br/com/androidcore/activity/CustomActivity

Any ideas? Should i shadow it? Should i put a reference into some Robolectric Config?

Here is my app build file:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.2"

    defaultConfig {
        applicationId "br.com.boozeprice"
        minSdkVersion 14
        targetSdkVersion 21
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    defaultConfig {
        testInstrumentationRunner "com.google.android.apps.common.testing.testrunner.GoogleInstrumentationTestRunner"
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:21.0.3'
    compile 'com.android.support:support-v4:21.0.3'

    compile project(':androidcore')
    compile project(':persistencecore')

    testCompile 'junit:junit:4.12'
    testCompile 'org.robolectric:robolectric:2.4'
}

Thanks in advance