Unexpected error when writing a GribMessage key

[pp-mo]

It is, by design, possible to modify the content of a GribMessage by assigning to a named key with a section,
e.g. message.section[4][keyname] = value

This appears to work as intended if you first read the key. For example :

sec4 = message.sections[4]
old_param = sec4["parameterNumber"]
sec4["parameterNumber"] = 17

But, if you attempt to write a key before reading it, you hit this exception.

I think this probably arose because the code intends to stop you unintentionally creating a key which is not 'native' to the template,
i.e. it wants to allow you to write only to keys which 'exist'.
But it looks like a bug, and could clearly be fixed.