Tirex: Empty/Transparent (103 B) tiles for zoom levels 0-2 on own tileserver, zoom levels 3 and higher are fine

Based on mapnik, tirex and german mapnik style I have rendered tiles from a planet file. All is fine for the zoom level 3 and up. But for the zoom levels 0 to 2 there are only blank tiles, that is, the PNGs are all empty/transparent and of size 103 Bytes.

Is this normal? Looking at the map at http://www.openstreetmap.de/karte.html?zoom=2&lat=41.93455&lon=-8.64009&layers=B000TT it should be possible to have a zoom level 2. Actually, I only need this one, the zoom 0 and 1 is not really needed.

Update: This seems to be a tirex problem. Rendering zoom level 2 with generate_tiles.py works fine.