I'm getting the same error.

On looking into the lib_dirs_for function in gem_path_searcher.rb I see 
it's trying to do a join against spec.require_paths.  Printing out the 
values I see the last entry is "." not ["."] as it should be. 
Presumably some gem has incorrectly set this value, but I don't know 
which one where or how.  I don't know enough about gems to solve it 
myself.  I put the following code in to try and get past this error:

def lib_dirs_for(spec)
    if(spec.require_paths.class == Array)
      value = spec.require_paths.join(',')
    else
      value = spec.require_paths
    end

    "#{spec.full_gem_path}/{#{value}}"
 end

Which get's me past the error.  I can run gem update, but no updates 
have come down, which means no one has published the fix to the 
offending gem yet.

Any more clues would be appreciated.  Also if the author of the 
offending gem would please step forward for your flogging.

Mr. Joshua
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