Matlab 2009: Why does optimset('Display', 'on') now say "Field name must be a string vector"?

I've used matlab 2009 (since 2009!) and have used the optimization toolbox many times. I've never had this problem before. My first instinct was "path problem", but the error happens in the optimset.m file in folder funfun (where it was in 2009, not in "optimfun" as it is now).

EDU>> optimset('Display', 'on') ??? Error using ==> struct Field name must be a string vector.

Error in ==> optimset at 126 options = struct(structinput{:});