Tests raise error VipsForeignSave: ".raw" is not a known buffer format

[anitagraham]

Running the test suite I get this error (x6)

 test_0080_pdf to raw                                           ERROR (0.00s)
Minitest::UnexpectedError:         Vips::Error: VipsRegion: valid clipped to nothing
        VipsForeignSave: ".raw" is not a known buffer format
        
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/ruby-vips-2.1.2/lib/vips/image.rb:627:in `write_to_buffer'
            /private/var/www/dragonfly_libvips/lib/dragonfly_libvips/processors/encode.rb:44:in `call'
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/dragonfly-1.4.0/lib/dragonfly/job/process.rb:23:in `apply'
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/dragonfly-1.4.0/lib/dragonfly/job.rb:119:in `block in apply'
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/dragonfly-1.4.0/lib/dragonfly/job.rb:119:in `each'
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/dragonfly-1.4.0/lib/dragonfly/job.rb:119:in `apply'
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/dragonfly-1.4.0/lib/dragonfly/job.rb:250:in `result'
            /Users/anita/.rbenv/versions/2.7.2/lib/ruby/2.7.0/forwardable.rb:229:in `mime_type'
            /private/var/www/dragonfly_libvips/test/dragonfly_libvips/processors/encode_test.rb:20:in `block (5 levels) in <top (required)>'

I can't see what/why it is happening.
In some instances the error "valid clipped to nothing" is repeated many times - perhaps that is the real problem.