diff --git a/langchain/src/output_parsers/json.ts b/langchain/src/output_parsers/json.ts
index da1bdd735e07..41c25c1a1476 100644
--- a/langchain/src/output_parsers/json.ts
+++ b/langchain/src/output_parsers/json.ts
@@ -2,6 +2,11 @@
 // MIT License
 
 export function parsePartialJson(s: string) {
+  // If the input is undefined, return null to indicate failure.
+  if (typeof s === "undefined") {
+    return null;
+  }
+
   // Attempt to parse the string as-is.
   try {
     return JSON.parse(s);