uninitialized constant Stripe::JavascriptHelper

I am now all of a sudden getting this error running bin/dev
after adding the mailpace gem.

This is being triggered on 
/Users/bryan.stewart/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/stripe-rails-2.3.4/lib/stripe/engine.rb:86:in `block (2 levels) in <class:Engine>': uninitialized constant Stripe::JavascriptHelper (NameError)

web    | 

web    |           helper Stripe::JavascriptHelper

has anyone experienced this before?

ok, I have confirmed that adding the 

gem 'mailpace-rails'

Is causing the above code to bomb on running the server.