diff --git a/pytools/prefork.py b/pytools/prefork.py
index 4f477b805f26b6c6ab67235f5330bed87c0793ad..49333371daf5120b371ffa4e422f9d2fe3f113d6 100644
--- a/pytools/prefork.py
+++ b/pytools/prefork.py
@@ -70,8 +70,8 @@ def _recv_packet(sock, who="Process", partner="other end"):
 
     if len(size_bytes) < size_bytes_size:
         from warnings import warn
-        warn("%s exiting upon apparent death of %"
-                % (who, partner))
+        warn("%s exiting upon apparent death of %s" % (who, partner))
+
         raise SystemExit
 
     size, = unpack("I", size_bytes)