diff --git a/tools/cpudist.py b/tools/cpudist.py
index 6a0e04b..40ccfa0 100755
--- a/tools/cpudist.py
+++ b/tools/cpudist.py
@@ -82,6 +82,11 @@
     if (tsp == 0)
         return;
 
+    if (ts < *tsp) {
+        // Probably a clock issue where the recorded on-CPU event had a
+        // timestamp later than the recorded off-CPU event, or vice versa.
+        return;
+    }
     u64 delta = ts - *tsp;
     FACTOR
     STORE