Detecting rails configuration failing with yaml error

[fumblehool]

Hi!

I'm trying to build rails app using v277 of this buildpack.
I'm getting this :-

Detecting rails configuration
    $ rails runner "begin; puts %Q{heroku.detecting.config.for.assets.compile=#{Rails.application.config.try(:assets).try(:compile)}}; rescue => e; puts e; puts e.backtrace; end; begin; puts %Q{heroku.detecting.config.for.action_dispatch.x_sendfile_header=#{Rails.application.config.try(:action_dispatch).try(:x_sendfile_header)}}; rescue => e; puts e; puts e.backtrace; end;"
    heroku.detecting.config.for.assets.compile=false
    heroku.detecting.config.for.action_dispatch.x_sendfile_header=
    
    yaml: line 3: mapping values are not allowed in this context
    ERROR: failed to build: exit status 3

It works with v270
I've double-checked, the .yml files are all valid.
Because its not printing: Detecting rails configuration failed, I think the detection command fails but I'm not able to verify this.

Can you please suggest what might be the problem?