diff --git a/js/libtextsecure.js b/js/libtextsecure.js
index c61c81110..b3e0db843 100644
--- a/js/libtextsecure.js
+++ b/js/libtextsecure.js
@@ -37718,6 +37718,9 @@ textsecure.MessageSender.prototype = {
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
+        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+            throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
+        }
         this.receiver = receiver;
 
         this.oncontact = this.onContactSyncComplete.bind(this);
diff --git a/libtextsecure/sync_request.js b/libtextsecure/sync_request.js
index 3d81039b9..bbb0dd850 100644
--- a/libtextsecure/sync_request.js
+++ b/libtextsecure/sync_request.js
@@ -8,6 +8,9 @@
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
+        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+            throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
+        }
         this.receiver = receiver;
 
         this.oncontact = this.onContactSyncComplete.bind(this);