diff --git a/fbchat/_util.py b/fbchat/_util.py
index ad411f2..bd96a3b 100644
--- a/fbchat/_util.py
+++ b/fbchat/_util.py
@@ -59,7 +59,7 @@ def parse_json(text: str) -> Any:
 
 
 def generate_offline_threading_id():
-    ret = _util.datetime_to_millis(datetime.datetime.utcnow())
+    ret = datetime_to_millis(datetime.datetime.utcnow())
     value = int(random.random() * 4294967295)
     string = ("0000000000000000000000" + format(value, "b"))[-22:]
     msgs = format(ret, "b") + string