Detection on stack failed [20180826 SSO/Io]

Probably no stars found on stack:

File ".../prose/prose/pipeline/photometry.py", line 94, in run
self.detection_s.run(show_progress=False)
...
File ".../prose/prose/blocks/detection.py", line 91, in single_detection
coordinates = np.transpose(np.array([sources["xcentroid"].data, sources["ycentroid"].data]))
TypeError: 'NoneType' object is not subscriptable

example on 20180826 SSO/Io