Agent + structured output + middleware error

I’ve created an middleware like this, which seems to be the same as the middleware in document. And it complains: Error: Invalid response from “wrapModelCall” in middleware “HandleModelErrors”: expected AIMessage, got object

const maxRetries = 3;

const handleModelErrors = createMiddleware({

name: "HandleModelErrors",

wrapModelCall: (request, handler) => {

for (let attempt = 0; attempt < maxRetries; attempt++) {

try {

return handler(request);

      } catch (e) {

if (attempt === maxRetries - 1) {

throw e;

        }

console.log(`Retry ${attempt + 1}/${maxRetries} after error: ${e}`);

      }

    }

throw new Error("Unreachable");

  },

})