A LangGraph StateGraph received a non-object return type from a node. Here’s an example: 
import { z } from "zod";
import { StateGraph } from "@langchain/langgraph";

const State = z.object({
  someKey: z.string(),
});

const badNode = (state: z.infer<typeof State>) => {
  // Should return an object with a value for "someKey", not an array
  return ["whoops"];
};

const builder = new StateGraph(State).addNode("badNode", badNode);
// ...

const graph = builder.compile();
Invoking the above graph will result in an error like this: 
await graph.invoke({ someKey: "someval" });

InvalidUpdateError: Expected object, got ['whoops']
For troubleshooting, visit: https://langchain-ai.github.io/langgraphjs/troubleshooting/errors/INVALID_GRAPH_NODE_RETURN_VALUE
Nodes in your graph must return an object containing one or more keys defined in your state.

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Troubleshooting

The following may help resolve this error:
  • If you have complex logic in your node, make sure all code paths return an appropriate object for your defined state.