diff --git a/Source/Port.m b/Source/Port.m
index 594dba48e..ada4c35a7 100644
--- a/Source/Port.m
+++ b/Source/Port.m
@@ -136,6 +136,7 @@
 
 - (void) setReceivedPacketInvocation: (id <Invoking>)invocation
 {
+  assert (!_packet_invocation);
   _packet_invocation = invocation;
 }