diff --git a/myems-bacnet/acquisition.py b/myems-bacnet/acquisition.py
index 58aa8ddf..5c7a4ade 100644
--- a/myems-bacnet/acquisition.py
+++ b/myems-bacnet/acquisition.py
@@ -510,7 +510,7 @@ def process(logger, ):
 
         if len(last_seen_data_source_set) > 0:
             update_row = (" UPDATE tbl_data_sources "
-                          " SET last_seen_datetime_utc = '%s' "
+                          " SET last_seen_datetime_utc = %s "
                           " WHERE id IN (")
 
             for data_source_id in last_seen_data_source_set: