diff --git a/Sources/CleverBird/chat/ChatCompletionResponse.swift b/Sources/CleverBird/chat/ChatCompletionResponse.swift
index 35d0b56..83584f4 100644
--- a/Sources/CleverBird/chat/ChatCompletionResponse.swift
+++ b/Sources/CleverBird/chat/ChatCompletionResponse.swift
@@ -4,7 +4,7 @@ import Foundation
 
 public struct Usage: Codable {
     public let promptTokens: Int
-    public let completionTokens: Int
+    public let completionTokens: Int?
     public let totalTokens: Int
 }