diff --git a/src/lib.rs b/src/lib.rs
index 9ac90e4..0ad745a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -25,7 +25,7 @@ pub struct Detail {
 pub struct Choice {
     pub delta: DeltaChoice,
     pub index: i64,
-    pub finish_reason: String,
+    pub finish_reason: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug)]