diff --git a/openbsc/tests/vty_test_runner.py b/openbsc/tests/vty_test_runner.py
index 3fbe20d..64fd86d 100644
--- a/openbsc/tests/vty_test_runner.py
+++ b/openbsc/tests/vty_test_runner.py
@@ -1239,6 +1239,8 @@
 
 def ipa_handle_small(x, verbose = False):
     s = data2str(x.recv(4))
+    if len(s) != 4*2:
+      raise Exception("expected to receive 4 bytes, but got %d (%r)" % (len(s)/2, s))
     if "0001fe00" == s:
         if (verbose):
             print "\tBSC <- NAT: PING?"