passing roi as geojson string doesn't work

Passing the ROI as a Geojson string results in an exception:

Traceback (most recent call last):
  File "/home/eggert/workspace/python/de-smart-monitoring-backend-module/module.py", line 20, in <module>
    d = downloader(roi=roi, average_spatial_values=True)
  File "/home/eggert/workspace/python/de-smart-monitoring-backend-module/de_sm_backend/nrt_io/downloader.py", line 191, in __init__
    self.roi=gpd.GeoDataFrame.from_features(roi)
  File "/home/eggert/workspace/python/de-smart-monitoring-backend-module/env/lib/python3.7/site-packages/geopandas/geodataframe.py", line 300, in from_features
    d = {"geometry": shape(f["geometry"]) if f["geometry"] else None}
TypeError: string indices must be integers

I passed this geojson string:

{ "type": "FeatureCollection",
          "features": [
            { "type": "Feature",
               "properties": {},
               "geometry": {
                 "type": "Polygon",
                 "coordinates": [
                   [ [11.5, 52.5], [12.5, 52.5], [12.5, 53.5],
                     [11.5, 53.5], [11.5, 52.5] ]
                   ]
               }
            }
          ]
      }

According to http://geojsonlint.com/ the mentioned string is valid.

Maybe the from_features method expects a dictionary instead of a string, try to parse the string via json.loads(roi) before passing it on.