diff --git a/Sources/OpenAI/Public/Models/ChatStreamResult.swift b/Sources/OpenAI/Public/Models/ChatStreamResult.swift
index 05052467..c1f30216 100644
--- a/Sources/OpenAI/Public/Models/ChatStreamResult.swift
+++ b/Sources/OpenAI/Public/Models/ChatStreamResult.swift
@@ -71,7 +71,7 @@ public struct ChatStreamResult: Codable, Equatable, Sendable {
 
             public struct ChoiceDeltaToolCall: Codable, Equatable, Sendable {
 
-                public let index: Int
+                public let index: Int?
                 /// The ID of the tool call.
                 public let id: String?
                 /// The function that the model called.