diff --git a/echo-client.rkt b/echo-client.rkt
index 689cffc..73a4eae 100644
--- a/echo-client.rkt
+++ b/echo-client.rkt
@@ -118,7 +118,7 @@
                                         (/ elapsed-sec count))))
 	    (write-logbook-datum! Tping (list (connection-count)
 					      roundtrip-latency
-					      (/ 1.0 roundtrip-latency)
+					      (if (zero? roundtrip-latency) 0 (/ 1.0 roundtrip-latency))
 					      count
 					      elapsed-sec)))))))
 
@@ -142,7 +142,7 @@
   (define gc-sec (/ gc-time 1000.0))
   (write-logbook-datum! Tgrowth (list old-count
 				      (/ wallclock-sec (- waypoint old-count))
-				      (/ (- waypoint old-count) wallclock-sec)
+				      (if (zero? wallclock-sec) 0 (/ (- waypoint old-count) wallclock-sec))
 				      cpu+gc-sec
 				      wallclock-sec
 				      gc-sec