Forum: Rails Engines Rails2, engine, undefined method `public_asset_directory'

I am trying to develop a Rails2 'engine', with the engines plugin.

Can an engine plugin not be a gem?  Everything works fine if my engines
plugin is actually a plugin. But if I try to make it a gem-plugin
instead, when trying to resolve assets in the plugin, I get this
following error. Which seems to have no google hits. Any hints?

undefined method `public_asset_directory'

/home/rochkind/empty/vendor/plugins/blacklight/vendor/plugins/engines/lib/engines/rails_extensions/asset_helpers.rb:112:in
`plugin_asset_path'
/home/rochkind/empty/vendor/plugins/blacklight/vendor/plugins/engines/lib/engines/rails_extensions/asset_helpers.rb:104:in
`pluginify_sources'
/home/rochkind/empty/vendor/plugins/blacklight/vendor/plugins/engines/lib/engines/rails_extensions/asset_helpers.rb:104:in
`map!'
/home/rochkind/empty/vendor/plugins/blacklight/vendor/plugins/engines/lib/engines/rails_extensions/asset_helpers.rb:104:in
`pluginify_sources'
/home/rochkind/empty/vendor/plugins/blacklight/vendor/plugins/engines/lib/engines/rails_extensions/asset_helpers.rb:72:in
`javascript_include_tag'

Wish you will be successful.