diff --git a/middleware/qira_program.py b/middleware/qira_program.py
index 10337402..293f6414 100644
--- a/middleware/qira_program.py
+++ b/middleware/qira_program.py
@@ -417,6 +417,7 @@ class Trace:
     self.strace = []
     self.mapped = []
 
+    self.keep_analysis_thread = True
     threading.Thread(target=self.analysis_thread).start()
 
   def fetch_raw_memory(self, clnum, address, ln):
@@ -502,8 +503,8 @@ class Trace:
     self.strace = ret
 
   def analysis_thread(self):
-    #print "*** started analysis_thread"
-    while 1:
+    print("*** started analysis_thread", self.forknum)
+    while self.keep_analysis_thread:
       time.sleep(0.2)
       # so this is done poorly, analysis can be incremental
       if self.maxclnum == None or self.db.get_maxclnum() != self.maxclnum:
@@ -527,6 +528,7 @@ class Trace:
         self.needs_update = True
 
         #print "analysis is ready"
+    print("*** ended analysis_thread", self.forknum)
 
   def load_base_memory(self):
     def get_forkbase_from_log(n):
diff --git a/middleware/qira_webserver.py b/middleware/qira_webserver.py
index b54ea8a8..4980a1bf 100644
--- a/middleware/qira_webserver.py
+++ b/middleware/qira_webserver.py
@@ -146,7 +146,9 @@ def deletefork(forknum):
   global program
   print("deletefork", forknum)
   os.unlink(qira_config.TRACE_FILE_BASE+str(int(forknum)))
-  del program.traces[forknum]
+  if forknum in program.traces:
+    program.traces[forknum].keep_analysis_thread = False
+    del program.traces[forknum]
   push_updates()
 
 @socketio.on('doslice', namespace='/qira')