Object is mistakenly treated as array

[f4rb3r1o]

I have this simple json object:

json obj, error;
error["error"] = "error_str";
obj["key"] = error;

That is to be deserialized as follow:
{"key": {"error":"error_str"}}

When I try to push this json into a std::vector:

std::vector artifacts;
artifacts.push_back(obj["key"]);

I get the following exception:

terminate called after throwing an instance of 'nlohmann::json_abi_v3_11_3::detail::type_error'
what(): [json.exception.type_error.305] cannot use operator[] with a string argument with array

I did some tests and apparently the inner json (ie. {"error":"error_str"}) is treated as an array.
My question is how can it possibly be treated that way? What am I doing wrong ?