[Orgmode] 6.29c breaks my agenda weekly view

[Bruce Hackett]

I have just installed v 6.29c and find that the agenda weekly view (C-c 
a a) fails with the following message:

Wrong type argument: stringp, nil

If I revert to my previous version 6.28e, the agenda view works 
properly. I have not changed anything else. This indicates that the 
problem arose between 6.28e and 6.29c.

I have looked through the gmane threads and the 6.29 release notes from 
Carsten, but I cannot find anything that would explain this (I may well 
have overlooked something). I guess this is a trivial problem, but it's 
beyond me.

Thanks for any help,
Bruce Hackett