diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index bc112b62..07096be7 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="at.bitfire.davdroid"
-    android:versionCode="18"
-    android:versionName="0.5-alpha" >
+    android:versionCode="19"
+    android:versionName="0.5.1-alpha" >
 
     <uses-sdk
         android:minSdkVersion="14"
diff --git a/README.md b/README.md
index d7646152..91884150 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ USED THIRD-PARTY LIBRARIES
 ==========================
 
 * [iCal4j](http://ical4j.sourceforge.net/) ([New BSD License](http://sourceforge.net/p/ical4j/ical4j/ci/default/tree/LICENSE))
-* [iCal4j-vcard](http://wiki.modularity.net.au/ical4j/index.php?title=VCard) ([New BSD License](http://sourceforge.net/p/ical4j/ical4j/ci/default/tree/LICENSE))
+* [ez-vcard](https://code.google.com/p/ez-vcard/) ([New BSD License](http://opensource.org/licenses/BSD-3-Clause))
 * [Simple XML Serialization](http://simple.sourceforge.net/) ([Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0))
 * [Project Lombok](http://projectlombok.org/) ([MIT License](http://opensource.org/licenses/mit-license.php))
 
diff --git a/src/at/bitfire/davdroid/Constants.java b/src/at/bitfire/davdroid/Constants.java
index 1031cf4b..6b415537 100644
--- a/src/at/bitfire/davdroid/Constants.java
+++ b/src/at/bitfire/davdroid/Constants.java
@@ -9,7 +9,7 @@ package at.bitfire.davdroid;
 
 public class Constants {
 	public static final String
-		APP_VERSION = "0.5-alpha",
+		APP_VERSION = "0.5.1-alpha",
 		
 		ACCOUNT_TYPE = "bitfire.at.davdroid",
 		
diff --git a/src/at/bitfire/davdroid/resource/Contact.java b/src/at/bitfire/davdroid/resource/Contact.java
index fd7ea591..dec3d141 100644
--- a/src/at/bitfire/davdroid/resource/Contact.java
+++ b/src/at/bitfire/davdroid/resource/Contact.java
@@ -38,7 +38,6 @@ import ezvcard.property.Nickname;
 import ezvcard.property.Note;
 import ezvcard.property.Organization;
 import ezvcard.property.Photo;
-import ezvcard.property.ProductId;
 import ezvcard.property.RawProperty;
 import ezvcard.property.Revision;
 import ezvcard.property.Role;
@@ -258,7 +257,7 @@ public class Contact extends Resource {
 		if (birthDay != null)
 			vcard.setBirthday(birthDay);
 
-		vcard.setProdId(new ProductId("DAVdroid/" + Constants.APP_VERSION));
+		vcard.setProdId("DAVdroid/" + Constants.APP_VERSION);
 		vcard.setRevision(Revision.now());
 		return Ezvcard
 			.write(vcard)
diff --git a/src/at/bitfire/davdroid/resource/Event.java b/src/at/bitfire/davdroid/resource/Event.java
index 10f2dbfb..80610cd8 100644
--- a/src/at/bitfire/davdroid/resource/Event.java
+++ b/src/at/bitfire/davdroid/resource/Event.java
@@ -42,6 +42,7 @@ import net.fortuna.ical4j.model.property.DtStart;
 import net.fortuna.ical4j.model.property.Duration;
 import net.fortuna.ical4j.model.property.ExDate;
 import net.fortuna.ical4j.model.property.ExRule;
+import net.fortuna.ical4j.model.property.LastModified;
 import net.fortuna.ical4j.model.property.Location;
 import net.fortuna.ical4j.model.property.Organizer;
 import net.fortuna.ical4j.model.property.ProdId;
@@ -216,6 +217,7 @@ public class Event extends Resource {
 		
 		event.getAlarms().addAll(alarms);
 		
+		props.add(new LastModified());
 		ical.getComponents().add(event);
 
 		// add VTIMEZONE components
@@ -296,6 +298,8 @@ public class Event extends Resource {
 	}
 
 	protected boolean hasNoTime(DateProperty date) {
+		if (date == null)
+			return false;
 		return !(date.getDate() instanceof DateTime);
 	}