diff --git a/src/gateway/openai-http.ts b/src/gateway/openai-http.ts
index 1f37dfb1f..d23fc64bf 100644
--- a/src/gateway/openai-http.ts
+++ b/src/gateway/openai-http.ts
@@ -526,7 +526,7 @@ export async function handleOpenAiHttpRequest(
     }
 
     if (evt.stream === "assistant") {
-      const content = resolveAssistantStreamDeltaText(evt);
+      const content = resolveAssistantStreamDeltaText(evt) ?? "";
       if (!content) {
         return;
       }