diff --git a/tastytrade/metrics.py b/tastytrade/metrics.py
index 77b3876..41f01e7 100644
--- a/tastytrade/metrics.py
+++ b/tastytrade/metrics.py
@@ -31,7 +31,7 @@ class Liquidity(TastytradeJsonDataclass):
     sum: Decimal
     count: int
     started_at: datetime
-    updated_at: datetime
+    updated_at: Optional[datetime] = None
 
 
 class OptionExpirationImpliedVolatility(TastytradeJsonDataclass):