diff --git a/mix/mix_protocol.nim b/mix/mix_protocol.nim
index 21efd60..91a6c6f 100644
--- a/mix/mix_protocol.nim
+++ b/mix/mix_protocol.nim
@@ -201,6 +201,9 @@ proc anonymizeLocalProtocolSend*(
   let index = pubNodeInfoKeys.find(destPeerId)
   if index != -1:
     availableIndices.del(index)
+  else:
+    error "Destination does not support Mix"
+    return
 
   for i in 0 ..< L:
     if i == L - 1: