Failed to get Teaspoon result object on page:

I'm trying to upgrade teaspoon on a web Ruby on Rails 5.2.3 app from 1.1.5 to 2.0. Teaspoon was working on 1.1.5 but when I upgraded it I get the following error;

% bundle exec teaspoon
Starting the Teaspoon server...
Teaspoon running default suite at http://127.0.0.1:35823/teaspoon/default
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-map7'
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-map7'
Failed to get Teaspoon result object on page: http://127.0.0.1:35823/teaspoon/default?reporter=Console 
   The title of this page was 'NoMethodError at /default'. 
    
   NoMethodError at /default
   undefined method `call' for nil:NilClass
   Application Frames All Frames
   Sprockets::CachedEnvironment#processors_for  sprockets (3.7.2) lib/sprockets/processing.rb, line 175
   Sprockets::CachedEnvironment#load_from_unloaded  sprockets (3.7.2) lib/sprockets/loader.rb, line 127
   block in Sprockets::CachedEnvironment#load  sprockets (3.7.2) lib/sprockets/loader.rb, line 60
   Sprockets::CachedEnvironment#fetch_asset_from_dependency_cache sprockets (3.7.2) lib/sprockets/loader.rb, line 317
   Sprockets::CachedEnvironment#load  sprockets (3.7.2) lib/sprockets/loader.rb, line 44
   block in Sprockets::CachedEnvironment#initialize sprockets (3.7.2) lib/sprockets/cached_environment.rb, line 20
   Sprockets::CachedEnvironment#load  sprockets (3.7.2) lib/sprockets/cached_environment.rb, line 47
... (further lines have been removed)

Failed to get Teaspoon result object on page: http://127.0.0.1:35823/teaspoon/default?reporter=Console 
   The title of this page was 'NoMethodError at /default'. 
    
   NoMethodError at /default
   undefined method `call' for nil:NilClass
   Application Frames All Frames
   Sprockets::CachedEnvironment#processors_for  sprockets (3.7.2) lib/sprockets/processing.rb, line 175
   Sprockets::CachedEnvironment#load_from_unloaded  sprockets (3.7.2) lib/sprockets/loader.rb, line 127
   block in Sprockets::CachedEnvironment#load  sprockets (3.7.2) lib/sprockets/loader.rb, line 60
   Sprockets::CachedEnvironment#fetch_asset_from_dependency_cache sprockets (3.7.2) lib/sprockets/loader.rb, line 317
   Sprockets::CachedEnvironment#load  sprockets (3.7.2) lib/sprockets/loader.rb, line 44
   block in Sprockets::CachedEnvironment#initialize sprockets (3.7.2) lib/sprockets/cached_environment.rb, line 20
   Sprockets::CachedEnvironment#load  sprockets (3.7.2) lib/sprockets/cached_environment.rb, line 47
... (further lines have been removed)

I'm using PhantomJS still as my backend. I've tried rerunning the generator to install everything and get the same problem.