diff --git a/script/TestTimestampTable.py b/script/TestTimestampTable.py
index 89c0f36..d6365dc 100644
--- a/script/TestTimestampTable.py
+++ b/script/TestTimestampTable.py
@@ -7,7 +7,7 @@ def after(rec, scan):
     path = get_exec_pars().group+"/TimeIOC"
     if rec.index==0:
         create_dataset(path, 's')
-    time_str += '.' + str(datetime.datetime.fromtimestamp(cache.nanos/1e9)).zfill(9)
+    time_str = '.' + str(datetime.datetime.fromtimestamp(cache.nanos/1e9)).zfill(9)
     append_dataset(path, time_str , type = 's')
 
 mscan(bpm6a, bpm6a, -1, 10.0, after_read = after)