m := map[string]interface{}{
  "field": map[string]interface{}{
    "sub-field": "hello",
  },
}
value, err := jsonpath.Get("$.field['sub-field']", m) 
// err is incorrectly set to `could not parse string: invalid syntax`

// the following case works correctly:
value, err := jsonpath.Get("$.field[\"sub-field\"]", m) 
// err is nil, value is "hello"