diff --git a/common/src/main/java/envoy/data/Attachment.java b/common/src/main/java/envoy/data/Attachment.java
index 0be0e1f..fc79d9f 100644
--- a/common/src/main/java/envoy/data/Attachment.java
+++ b/common/src/main/java/envoy/data/Attachment.java
@@ -1,6 +1,7 @@
 package envoy.data;
 
 import java.io.Serializable;
+import java.util.Objects;
 
 /**
  * This interface should be used for any type supposed to be a {@link Message} attachment (i.e.
@@ -63,9 +64,9 @@ public final class Attachment implements Serializable {
 	 * @since Envoy Common v0.1-beta
 	 */
 	public Attachment(byte[] data, String name, AttachmentType type) {
-		this.data	= data;
-		this.name	= name;
-		this.type	= type;
+		this.data	= Objects.requireNonNull(data);
+		this.name	= Objects.requireNonNull(name);
+		this.type	= Objects.requireNonNull(type);
 	}
 
 	/**
diff --git a/common/src/main/java/envoy/data/Contact.java b/common/src/main/java/envoy/data/Contact.java
index 8e250dc..18e0e4f 100644
--- a/common/src/main/java/envoy/data/Contact.java
+++ b/common/src/main/java/envoy/data/Contact.java
@@ -29,8 +29,8 @@ public abstract class Contact implements Serializable {
 	 */
 	public Contact(long id, String name, Set<? extends Contact> contacts) {
 		this.id			= id;
-		this.name		= name;
-		this.contacts	= contacts;
+		this.name		= Objects.requireNonNull(name);
+		this.contacts	= contacts == null ? new HashSet<>() : contacts;
 	}
 
 	/**
diff --git a/common/src/main/java/envoy/data/GroupMessage.java b/common/src/main/java/envoy/data/GroupMessage.java
index d6a30a9..8f2f28a 100644
--- a/common/src/main/java/envoy/data/GroupMessage.java
+++ b/common/src/main/java/envoy/data/GroupMessage.java
@@ -38,7 +38,8 @@ public final class GroupMessage extends Message {
 		Map<Long, MessageStatus> memberStatuses) {
 		super(id, senderID, groupID, creationDate, receivedDate, readDate, text, attachment, status,
 			forwarded);
-		this.memberStatuses = memberStatuses;
+		this.memberStatuses =
+			memberStatuses == null ? new HashMap<>() : memberStatuses;
 	}
 
 	/**
diff --git a/common/src/main/java/envoy/data/LoginCredentials.java b/common/src/main/java/envoy/data/LoginCredentials.java
index f121005..c0a455a 100644
--- a/common/src/main/java/envoy/data/LoginCredentials.java
+++ b/common/src/main/java/envoy/data/LoginCredentials.java
@@ -2,6 +2,7 @@ package envoy.data;
 
 import java.io.Serializable;
 import java.time.Instant;
+import java.util.Objects;
 
 /**
  * Contains a {@link User}'s login / registration information as well as the client version.
@@ -22,12 +23,12 @@ public final class LoginCredentials implements Serializable {
 	private LoginCredentials(String identifier, String password, boolean registration,
 		boolean token, String clientVersion,
 		Instant lastSync) {
-		this.identifier		= identifier;
-		this.password		= password;
+		this.identifier		= Objects.requireNonNull(identifier);
+		this.password		= Objects.requireNonNull(password);
 		this.registration	= registration;
 		this.token			= token;
-		this.clientVersion	= clientVersion;
-		this.lastSync		= lastSync;
+		this.clientVersion	= Objects.requireNonNull(clientVersion);
+		this.lastSync		= lastSync == null ? Instant.EPOCH : lastSync;
 	}
 
 	/**
diff --git a/common/src/main/java/envoy/data/Message.java b/common/src/main/java/envoy/data/Message.java
index b8965a3..78a5ad6 100644
--- a/common/src/main/java/envoy/data/Message.java
+++ b/common/src/main/java/envoy/data/Message.java
@@ -2,6 +2,7 @@ package envoy.data;
 
 import java.io.Serializable;
 import java.time.Instant;
+import java.util.Objects;
 
 import dev.kske.eventbus.IEvent;
 
@@ -80,9 +81,9 @@ public class Message implements Serializable, IEvent {
 		this.creationDate	= creationDate;
 		this.receivedDate	= receivedDate;
 		this.readDate		= readDate;
-		this.text			= text;
+		this.text			= text == null ? "" : text;
 		this.attachment		= attachment;
-		this.status			= status;
+		this.status			= Objects.requireNonNull(status);
 		this.forwarded		= forwarded;
 	}
 
diff --git a/common/src/main/java/envoy/data/User.java b/common/src/main/java/envoy/data/User.java
index f0f2a62..6b7c580 100644
--- a/common/src/main/java/envoy/data/User.java
+++ b/common/src/main/java/envoy/data/User.java
@@ -86,7 +86,7 @@ public final class User extends Contact {
 	 */
 	public User(long id, String name, UserStatus status, Set<Contact> contacts) {
 		super(id, name, contacts);
-		this.status = status;
+		this.status = Objects.requireNonNull(status);
 	}
 
 	@Override
diff --git a/common/src/main/java/envoy/event/Event.java b/common/src/main/java/envoy/event/Event.java
index e12b882..5812878 100644
--- a/common/src/main/java/envoy/event/Event.java
+++ b/common/src/main/java/envoy/event/Event.java
@@ -1,6 +1,7 @@
 package envoy.event;
 
 import java.io.Serializable;
+import java.util.Objects;
 
 import dev.kske.eventbus.IEvent;
 
@@ -20,7 +21,16 @@ public abstract class Event<T> implements IEvent, Serializable {
 	private static final long serialVersionUID = 0L;
 
 	protected Event(T value) {
-		this.value = value;
+		this(value, false);
+	}
+
+	/**
+	 * This constructor is reserved for {@link Valueless} events. No other event should contain null
+	 * values. Only use if really necessary. Using this constructor with {@code true} implies that
+	 * the user has to manually check if the value of the event is null.
+	 */
+	protected Event(T value, boolean canBeNull) {
+		this.value = canBeNull ? value : Objects.requireNonNull(value);
 	}
 
 	/**
@@ -46,7 +56,7 @@ public abstract class Event<T> implements IEvent, Serializable {
 		private static final long serialVersionUID = 0L;
 
 		protected Valueless() {
-			super(null);
+			super(null, true);
 		}
 
 		@Override
diff --git a/common/src/main/java/envoy/event/GroupCreationResult.java b/common/src/main/java/envoy/event/GroupCreationResult.java
index c1a002b..2c01b44 100644
--- a/common/src/main/java/envoy/event/GroupCreationResult.java
+++ b/common/src/main/java/envoy/event/GroupCreationResult.java
@@ -20,7 +20,7 @@ public class GroupCreationResult extends Event<Group> {
 	 * @since Envoy Common v0.2-beta
 	 */
 	public GroupCreationResult() {
-		super(null);
+		super(null, true);
 	}
 
 	/**
diff --git a/common/src/main/java/envoy/event/GroupResize.java b/common/src/main/java/envoy/event/GroupResize.java
index 69e0447..3679f14 100644
--- a/common/src/main/java/envoy/event/GroupResize.java
+++ b/common/src/main/java/envoy/event/GroupResize.java
@@ -2,6 +2,8 @@ package envoy.event;
 
 import static envoy.event.ElementOperation.*;
 
+import java.util.Objects;
+
 import envoy.data.*;
 
 /**
@@ -30,7 +32,7 @@ public final class GroupResize extends Event<User> {
 	 */
 	public GroupResize(User user, Group group, ElementOperation operation) {
 		super(user);
-		this.operation = operation;
+		this.operation = Objects.requireNonNull(operation);
 		final var contained = group.getContacts().contains(user);
 		if (contained && operation.equals(ADD))
 			throw new IllegalArgumentException(String.format("Cannot add %s to %s!", user, group));
diff --git a/common/src/main/java/envoy/event/IsTyping.java b/common/src/main/java/envoy/event/IsTyping.java
index 4c5e93e..4e587b4 100644
--- a/common/src/main/java/envoy/event/IsTyping.java
+++ b/common/src/main/java/envoy/event/IsTyping.java
@@ -28,7 +28,7 @@ public final class IsTyping extends Event<Long> {
 	 * @param destinationID the ID of the contact the user wrote to
 	 * @since Envoy Common v0.2-beta
 	 */
-	public IsTyping(Long sourceID, long destinationID) {
+	public IsTyping(long sourceID, long destinationID) {
 		super(sourceID);
 		this.destinationID = destinationID;
 	}
diff --git a/common/src/main/java/envoy/event/IssueProposal.java b/common/src/main/java/envoy/event/IssueProposal.java
index d015e88..0b2cc5f 100644
--- a/common/src/main/java/envoy/event/IssueProposal.java
+++ b/common/src/main/java/envoy/event/IssueProposal.java
@@ -23,7 +23,7 @@ public final class IssueProposal extends Event<String> {
 	 */
 	public IssueProposal(String title, String description, boolean isBug) {
 		super(escape(title));
-		this.description	= sanitizeDescription(description);
+		this.description	= description == null ? "" : sanitizeDescription(description);
 		bug					= isBug;
 	}
 
@@ -37,8 +37,8 @@ public final class IssueProposal extends Event<String> {
 	 */
 	public IssueProposal(String title, String description, String user, boolean isBug) {
 		super(escape(title));
-		this.description	=
-			sanitizeDescription(description) + String.format("<br>Submitted by user %s.", user);
+		this.description	= description == null ? ""
+			: sanitizeDescription(description) + String.format("<br>Submitted by user %s.", user);
 		bug					= isBug;
 	}
 
diff --git a/common/src/main/java/envoy/event/MessageStatusChange.java b/common/src/main/java/envoy/event/MessageStatusChange.java
index 5bdacde..5581f67 100644
--- a/common/src/main/java/envoy/event/MessageStatusChange.java
+++ b/common/src/main/java/envoy/event/MessageStatusChange.java
@@ -1,6 +1,7 @@
 package envoy.event;
 
 import java.time.Instant;
+import java.util.Objects;
 
 import envoy.data.Message;
 
@@ -26,7 +27,7 @@ public class MessageStatusChange extends Event<Message.MessageStatus> {
 	public MessageStatusChange(long id, Message.MessageStatus status, Instant date) {
 		super(status);
 		this.id		= id;
-		this.date	= date;
+		this.date	= Objects.requireNonNull(date);
 	}
 
 	/**
diff --git a/common/src/main/java/envoy/event/PasswordChangeRequest.java b/common/src/main/java/envoy/event/PasswordChangeRequest.java
index b1b860c..023c482 100644
--- a/common/src/main/java/envoy/event/PasswordChangeRequest.java
+++ b/common/src/main/java/envoy/event/PasswordChangeRequest.java
@@ -1,5 +1,7 @@
 package envoy.event;
 
+import java.util.Objects;
+
 import envoy.data.Contact;
 
 /**
@@ -21,7 +23,7 @@ public final class PasswordChangeRequest extends Event<String> {
 	 */
 	public PasswordChangeRequest(String newPassword, String oldPassword, long userID) {
 		super(newPassword);
-		this.oldPassword	= oldPassword;
+		this.oldPassword	= Objects.requireNonNull(oldPassword);
 		id					= userID;
 	}
 
diff --git a/common/src/main/java/envoy/event/contact/UserOperation.java b/common/src/main/java/envoy/event/contact/UserOperation.java
index bbec1c9..fd4de7f 100644
--- a/common/src/main/java/envoy/event/contact/UserOperation.java
+++ b/common/src/main/java/envoy/event/contact/UserOperation.java
@@ -1,5 +1,7 @@
 package envoy.event.contact;
 
+import java.util.Objects;
+
 import envoy.data.User;
 import envoy.event.*;
 
@@ -24,7 +26,7 @@ public final class UserOperation extends Event<User> {
 	 */
 	public UserOperation(User contact, ElementOperation operationType) {
 		super(contact);
-		this.operationType = operationType;
+		this.operationType = Objects.requireNonNull(operationType);
 	}
 
 	/**
diff --git a/server/src/main/java/envoy/server/data/PersistenceManager.java b/server/src/main/java/envoy/server/data/PersistenceManager.java
index ddabe7b..d2e1f22 100755
--- a/server/src/main/java/envoy/server/data/PersistenceManager.java
+++ b/server/src/main/java/envoy/server/data/PersistenceManager.java
@@ -1,7 +1,7 @@
 package envoy.server.data;
 
 import java.time.Instant;
-import java.util.List;
+import java.util.*;
 import java.util.logging.Level;
 
 import javax.persistence.*;
@@ -223,6 +223,9 @@ public final class PersistenceManager {
 	 * @since Envoy Server Standalone v0.2-beta
 	 */
 	public List<Message> getPendingMessages(User user, Instant lastSync) {
+		if (user == null)
+			return new ArrayList<>();
+		lastSync = Objects.requireNonNullElse(lastSync, Instant.EPOCH);
 		return entityManager.createNamedQuery(Message.getPending).setParameter("user", user)
 			.setParameter("lastSeen", lastSync).getResultList();
 	}
@@ -236,6 +239,9 @@ public final class PersistenceManager {
 	 * @since Envoy Server Standalone v0.2-beta
 	 */
 	public List<GroupMessage> getPendingGroupMessages(User user, Instant lastSync) {
+		if (user == null)
+			return new ArrayList<>();
+		lastSync = Objects.requireNonNullElse(lastSync, Instant.EPOCH);
 		return entityManager.createNamedQuery(GroupMessage.getPendingGroupMsg)
 			.setParameter("userId", user.getID())
 			.setParameter("lastSeen", lastSync)
@@ -277,16 +283,18 @@ public final class PersistenceManager {
 	 * @since Envoy Server v0.3-beta
 	 */
 	public void addContactBidirectional(Contact contact1, Contact contact2) {
+		if (!(contact1 == null || contact2 == null)) {
 
-		// Add users to each others contact list
-		contact1.getContacts().add(contact2);
-		contact2.getContacts().add(contact1);
+			// Add users to each others contact list
+			contact1.getContacts().add(contact2);
+			contact2.getContacts().add(contact1);
 
-		// Synchronize changes with the database
-		transaction(() -> {
-			entityManager.merge(contact1);
-			entityManager.merge(contact2);
-		});
+			// Synchronize changes with the database
+			transaction(() -> {
+				entityManager.merge(contact1);
+				entityManager.merge(contact2);
+			});
+		}
 	}
 
 	/**
@@ -308,16 +316,18 @@ public final class PersistenceManager {
 	 * @since Envoy Server v0.3-beta
 	 */
 	public void removeContactBidirectional(Contact contact1, Contact contact2) {
+		if (!(contact1 == null || contact2 == null)) {
 
-		// Remove users from each others contact list
-		contact1.getContacts().remove(contact2);
-		contact2.getContacts().remove(contact1);
+			// Remove users from each others contact list
+			contact1.getContacts().remove(contact2);
+			contact2.getContacts().remove(contact1);
 
-		// Synchronize changes with the database
-		transaction(() -> {
-			entityManager.merge(contact1);
-			entityManager.merge(contact2);
-		});
+			// Synchronize changes with the database
+			transaction(() -> {
+				entityManager.merge(contact1);
+				entityManager.merge(contact2);
+			});
+		}
 	}
 
 	/**