Hi I think the wrann method inside wfdb.io.Annotation might not be working as intended right now.

When I try to write a new annotation with only the label_store field set, I get a Nonetype error inside wfdb.io.Annotation.calc_core_bytes (line 930) where it says data_bytes.append(field2bytes('samptype', [sampdiff[i], self.symbol[i]])), because self.symbol is None (and thus unsubscriptable).

From the documentation it should however also be possible to only use label_store as a field in my Annotation.