[isabelle] I/O error after "Isabelle build -o browser_info -v -c -a"

I have Ubuntu Linux 12.04 and downloaded Isabelle 2013 Feb.

I do this:
../Isabelle2013/bin/isabelle build -o browser_info -v -c -a

and everything seems fine. I even get fine looking HTML pages in places like

But for ZF I get errors:
### I/O error: /home/reza/.isabelle/Isabelle2013/browser_info/ZF/.session/entries (No such file or directory) ### Browser info: failed to update session index of "$ISABELLE_BROWSER_INFO/ZF"

Can you help please.

Thank you,

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