diff --git a/briar-core/src/main/java/org/briarproject/briar/client/MessageTreeImpl.java b/briar-core/src/main/java/org/briarproject/briar/client/MessageTreeImpl.java
index 05c345dbc..8176e3969 100644
--- a/briar-core/src/main/java/org/briarproject/briar/client/MessageTreeImpl.java
+++ b/briar-core/src/main/java/org/briarproject/briar/client/MessageTreeImpl.java
@@ -12,6 +12,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
+import javax.annotation.concurrent.GuardedBy;
 import javax.annotation.concurrent.ThreadSafe;
 
 @ThreadSafe
@@ -19,10 +20,16 @@ import javax.annotation.concurrent.ThreadSafe;
 public class MessageTreeImpl<T extends MessageTree.MessageNode>
 		implements MessageTree<T> {
 
+	@GuardedBy("this")
 	private final Map<MessageId, List<T>> nodeMap = new HashMap<>();
+
+	@GuardedBy("this")
 	private final List<T> roots = new ArrayList<>();
+
+	@GuardedBy("this")
 	private final List<List<T>> unsortedLists = new ArrayList<>();
 
+	@SuppressWarnings("UseCompareMethod")
 	private Comparator<T> comparator = (o1, o2) ->
 			Long.valueOf(o1.getTimestamp()).compareTo(o2.getTimestamp());
 
@@ -50,11 +57,13 @@ public class MessageTreeImpl<T extends MessageTree.MessageNode>
 		add(Collections.singletonList(node));
 	}
 
+	@GuardedBy("this")
 	private void markAsUnsorted(List<T> list) {
 		if (!unsortedLists.contains(list))
 			unsortedLists.add(list);
 	}
 
+	@GuardedBy("this")
 	private void parseNode(T node) {
 		if (node.getParentId() == null) {
 			roots.add(node);
@@ -67,6 +76,7 @@ public class MessageTreeImpl<T extends MessageTree.MessageNode>
 		}
 	}
 
+	@GuardedBy("this")
 	private void sortUnsorted() {
 		for (List<T> list : unsortedLists) {
 			Collections.sort(list, comparator);
@@ -74,6 +84,7 @@ public class MessageTreeImpl<T extends MessageTree.MessageNode>
 		unsortedLists.clear();
 	}
 
+	@GuardedBy("this")
 	private void traverse(List<T> list, T node, int level) {
 		list.add(node);
 		List<T> children = nodeMap.get(node.getId());
@@ -103,7 +114,7 @@ public class MessageTreeImpl<T extends MessageTree.MessageNode>
 	}
 
 	@Override
-	public boolean contains(MessageId m) {
+	public synchronized boolean contains(MessageId m) {
 		return nodeMap.containsKey(m);
 	}
 }