diff --git a/app/build.gradle b/app/build.gradle
index cbfa2897..de50a1cb 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -122,6 +122,9 @@ android {
         buildTypes.release.signingConfig = null
     }
     compileOptions {
+        // enable because ical4android requires desugaring
+        coreLibraryDesugaringEnabled true
+
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
     }
@@ -135,6 +138,8 @@ android {
 
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
+    coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:1.0.5'
+
     implementation "org.jetbrains.anko:anko-commons:0.10.4"
 
     implementation "com.etesync:journalmanager:1.1.1"
diff --git a/app/proguard-rules.txt b/app/proguard-rules.txt
index b981122e..345ebb71 100644
--- a/app/proguard-rules.txt
+++ b/app/proguard-rules.txt
@@ -10,7 +10,6 @@
 -optimizations !code/simplification/arithmetic,!code/simplification/cast,!field/*,!class/merging/*
 -optimizationpasses 5
 -allowaccessmodification
--dontpreverify
 
 # Kotlin
 -dontwarn kotlin.**
@@ -25,14 +24,7 @@
 -keep class ezvcard.property.** { *; }  # keep all vCard properties (created at runtime)
 
 # ical4j: ignore unused dynamic libraries
--dontwarn aQute.**
--dontwarn groovy.**                       # Groovy-based ContentBuilder not used
--dontwarn javax.cache.**                  # no JCache support in Android
--dontwarn net.fortuna.ical4j.model.**
--dontwarn org.codehaus.groovy.**
--dontwarn org.apache.log4j.**             # ignore warnings from log4j dependency
 -keep class net.fortuna.ical4j.** { *; }  # keep all model classes (properties/factories, created at runtime)
--keep class org.threeten.bp.** { *; }     # keep ThreeTen (for time zone processing)
 
 # okhttp
 # JSR 305 annotations are for embedding nullability information.