diff --git a/hbs2-core/lib/HBS2/Prelude.hs b/hbs2-core/lib/HBS2/Prelude.hs
index 736ef084..d8ba546b 100644
--- a/hbs2-core/lib/HBS2/Prelude.hs
+++ b/hbs2-core/lib/HBS2/Prelude.hs
@@ -249,7 +249,8 @@ instance Probe SimpleProbe where
     t <- liftIO getPOSIXTime <&> round
     atomically do
       writeTVar spTimestamp t
-      modifyTVar spProbeValues (<> HM.fromList values)
+      old <- readTVar spProbeValues
+      writeTVar spProbeValues (HM.fromList values <> old)
 
 
 dontHandle :: Applicative f => a -> f ()