[O] debugging export error

[Charles Berry]

I am trying to debug a subtree export. 

I get 

member-ignore-case: Wrong type argument: stringp, nil

and it seems to come from `org-export-get-environment' but if I instrument
that function, the error goes away whether I step thru it or `c' on entry.

Likewise, if I M-x debug-on-entry and just `c' on entry, I get no error.

Bisecting the file with the subtree I am trying to export, I can find things
- like removing an unused src block - that when changed get rid of the 
error.

But I'd really like to know what is going on here and not being able to
see the problem thru a debugger is a frustration.

Advice appreciated.

Chuck