Calling a shared method produces a weird array out of bounds error

Question

A TypeScript client does not work for DFINITY Motoko:

function initializeDirectCanDBPartitionClient(canisterId: Principal)
{
  const host = isLocal ? "http://127.0.0.1:8000" : "https://ic0.app";
  const agent = new HttpAgent({ host });
  return Actor.createActor(CanDBPartitionIDL, { agent, canisterId });
};
...
const client = initializeDirectCanDBPartitionClient(obj.itemRef.canister);
obj.item = await client.getItem(BigInt(obj.itemRef.id)) as any;

produces

Uncaught (in promise) Error: Call failed:
  Canister: a3shf-5eaaa-aaaaa-qaafa-cai
  Method: getItem (query)
  "Status": "rejected"
  "Code": "CanisterError"
  "Message": "IC0503: Canister a3shf-5eaaa-aaaaa-qaafa-cai trapped explicitly: Array index out of bounds"
    at caller (index.js:184:27)
    at async ItemData.create (index.js:78705:20)

But

  public query func getItem(itemId: Nat): async ?lib.Item {
    Debug.print("ITEMXXX");
    return null; // to ensure the action is completely done
    let data = _getAttribute({sk = "i/" # Nat.toText(itemId)}, "i");
    Debug.print("ITEM: " # debug_show(data));
    do ? { lib.deserializeItem(data!) };
  };

should output at least ITEMXXX, which however does not appear in the logs of dfx start.

Why the error?!