Upgrading kennel gem isn't smooth

[epssy]

Running gem update on ruby-2.5.1 with rdoc-6.0.1 results in a fault.

Successfully installed kennel-1.28.3
Parsing documentation for kennel-1.28.3
Installing ri documentation for kennel-1.28.3
Installing darkfish documentation for kennel-1.28.3
ERROR:  While executing gem ... (RDoc::Error)
    error generating Kennel/Importer.html: no implicit conversion of nil into String (TypeError)

Workaround was to run the install without docs.

gem install kennel --no-rdoc --no-ri

[grosser]

never seen that :(
I'm using this in ~/.gemrc gem: "--no-ri --no-rdoc"

also unable to reproduce on ruby 2.5.1:

gem i kennel
Fetching: kennel-1.28.3.gem (100%)
Successfully installed kennel-1.28.3
Parsing documentation for kennel-1.28.3
Installing ri documentation for kennel-1.28.3
Done installing documentation for kennel after 0 seconds
1 gem installed

... try gem update --system
otherwise run with --backtrace to get more details