If an event has no tid or pid, dont add it to the tid or pid list
diff --git a/trace_event/trace_events.py b/trace_event/trace_events.py
index 240f0f3..fb1771e 100644
--- a/trace_event/trace_events.py
+++ b/trace_event/trace_events.py
@@ -52,7 +52,8 @@
       return self.pids
     pids = set()
     for e in self.events:
-      pids.add(e["pid"])
+      if "pid" in e and e["pid"]:
+        pids.add(e["pid"])
     self.pids = list(pids)
     return self.pids
 
@@ -61,7 +62,8 @@
       return self.tids
     tids = set()
     for e in self.events:
-      tids.add(e["tid"])
+      if "tid" in e and e["tid"]:
+        tids.add(e["tid"])
     self.tids = list(tids)
     return self.tids