The MDLV2000Writer assumes the property of the Atom_Atom_Mapping to be of type integer. Whereas this is a correct assumption within the SDFile format, properties are stored as objects and the Atom_Atom_Mapping could be a string as a result. E.g., when a atom id is used (a1) or the AtomContainer had been serialized somehow in between. The patch adds a conditional to cast the property into either a String or Integer. If the value cannot be parsed into an Integer, it gets ignored as it can not be stored according to the file format specifications.