diff --git a/smack-extensions/src/main/java/org/jivesoftware/smackx/blocking/element/BlockContactsIQ.java b/smack-extensions/src/main/java/org/jivesoftware/smackx/blocking/element/BlockContactsIQ.java
index 19ca0eb72..7a137668a 100644
--- a/smack-extensions/src/main/java/org/jivesoftware/smackx/blocking/element/BlockContactsIQ.java
+++ b/smack-extensions/src/main/java/org/jivesoftware/smackx/blocking/element/BlockContactsIQ.java
@@ -16,6 +16,7 @@
  */
 package org.jivesoftware.smackx.blocking.element;
 
+import java.util.Collections;
 import java.util.List;
 
 import org.jivesoftware.smack.packet.IQ;
@@ -51,7 +52,7 @@ public class BlockContactsIQ extends IQ {
     public BlockContactsIQ(List<Jid> jids) {
         super(ELEMENT, NAMESPACE);
         this.setType(Type.set);
-        this.jids = jids;
+        this.jids = Collections.unmodifiableList(jids);
     }
 
     /**