diff --git a/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt b/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt
index 6f0016ad..009f4743 100644
--- a/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt
+++ b/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt
@@ -37,6 +37,6 @@ data class GenerateContentResponse(
 @Serializable
 data class UsageMetadata(
   val promptTokenCount: Int,
-  val candidatesTokenCount: Int,
+  val candidatesTokenCount: Int?,
   val totalTokenCount: Int
 )