Error from inside ActionView::Template::Error - String can't be coerced into Integer

We recently updated the ruby and rubygems versions of our app, and we've been consistently getting the following error:

A TypeError occurred in [model]#[view]:

String can't be coerced into Integer
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_view/template/error.rb:92:in `+'

Here is the backtrace:

vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_view/template/error.rb:92:in `+'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_view/template/error.rb:92:in `sum'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_view/template/error.rb:92:in `source_extract'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_view/template/error.rb:112:in `annoted_source_code'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/debug_exceptions.rb:68:in `block in log_error'
vendor/bundle/ruby/2.5.0/gems/activesupport-3.2.22.5/lib/active_support/deprecation/reporting.rb:20:in `silence'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/debug_exceptions.rb:66:in `log_error'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/debug_exceptions.rb:34:in `render_exception'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/debug_exceptions.rb:27:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/exception_notification-4.1.1/lib/exception_notification/rack.rb:32:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/show_exceptions.rb:56:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/railties-3.2.22.5/lib/rails/rack/logger.rb:32:in `call_app'
vendor/bundle/ruby/2.5.0/gems/railties-3.2.22.5/lib/rails/rack/logger.rb:16:in `block in call'
vendor/bundle/ruby/2.5.0/gems/activesupport-3.2.22.5/lib/active_support/tagged_logging.rb:22:in `tagged'
vendor/bundle/ruby/2.5.0/gems/railties-3.2.22.5/lib/rails/rack/logger.rb:16:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/request_id.rb:22:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/rack-1.4.7/lib/rack/methodoverride.rb:21:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/rack-1.4.7/lib/rack/runtime.rb:17:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/activesupport-3.2.22.5/lib/active_support/cache/strategy/local_cache.rb:72:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/rack-1.4.7/lib/rack/lock.rb:15:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/rack-1.4.7/lib/rack/deflater.rb:13:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/actionpack-3.2.22.5/lib/action_dispatch/middleware/static.rb:83:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/rack-ssl-1.3.4/lib/rack/ssl.rb:27:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/rack-cache-1.7.2/lib/rack/cache/context.rb:140:in `forward'
vendor/bundle/ruby/2.5.0/gems/rack-cache-1.7.2/lib/rack/cache/context.rb:249:in `fetch'
vendor/bundle/ruby/2.5.0/gems/rack-cache-1.7.2/lib/rack/cache/context.rb:189:in `lookup'
vendor/bundle/ruby/2.5.0/gems/rack-cache-1.7.2/lib/rack/cache/context.rb:66:in `call!'
vendor/bundle/ruby/2.5.0/gems/rack-cache-1.7.2/lib/rack/cache/context.rb:51:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
lib/rack/assets.rb:167:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
lib/rack/basics.rb:24:in `call'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/railties-3.2.22.5/lib/rails/engine.rb:484:in `call'
vendor/bundle/ruby/2.5.0/gems/railties-3.2.22.5/lib/rails/application.rb:231:in `call'
vendor/bundle/ruby/2.5.0/gems/railties-3.2.22.5/lib/rails/railtie/configurable.rb:30:in `method_missing'
vendor/bundle/ruby/2.5.0/gems/newrelic_rpm-5.1.0.344/lib/new_relic/agent/instrumentation/middleware_tracing.rb:92:in `call'
vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:606:in `process_client'
config/unicorn.rb:94:in `process_client_with_gc'
vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:701:in `worker_loop'
vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:549:in `spawn_missing_workers'
vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:142:in `start'
vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/bin/unicorn:126:in `<top (required)>'
vendor/bundle/ruby/2.5.0/bin/unicorn:23:in `load'
vendor/bundle/ruby/2.5.0/bin/unicorn:23:in `<top (required)>'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/cli/exec.rb:75:in `load'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/cli/exec.rb:75:in `kernel_load'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/cli/exec.rb:28:in `run'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/cli.rb:424:in `exec'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/vendor/thor/lib/thor/invocation.rb:126:in `invoke_command'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/vendor/thor/lib/thor.rb:387:in `dispatch'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/cli.rb:27:in `dispatch'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/vendor/thor/lib/thor/base.rb:466:in `start'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/cli.rb:18:in `start'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/exe/bundle:30:in `block in <top (required)>'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/lib/bundler/friendly_errors.rb:122:in `with_friendly_errors'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/gems/bundler-1.16.1/exe/bundle:22:in `<top (required)>'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/bin/bundle:23:in `load'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/bin/bundle:23:in `<main>'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/bin/ruby_executable_hooks:15:in `eval'
/usr/local/rvm/gems/ruby-2.5.1-tagX4gpqkt9tzqft/bin/ruby_executable_hooks:15:in `<main>'

I'm trying to understand why this error is happening. Based on the backtrace it doesn't seem to be caused by our code, and since it only happens on some pages of the site I don't think that its a gem compatibility issue. Of note is that the ActionView::Template::Error code is calling sum on an array of strings:

79  def source_extract(indentation = 0)
80    return unless num = line_number
81    num = num.to_i
82
83    source_code = @template.source.split("\n")
84
85    start_on_line = [ num - SOURCE_CODE_RADIUS - 1, 0 ].max
86    end_on_line   = [ num + SOURCE_CODE_RADIUS - 1, source_code.length].min
87
88    indent = ' ' * indentation
89    line_counter = start_on_line
90    return unless source_code = source_code[start_on_line..end_on_line]
91
92    source_code.sum do |line|
93      line_counter += 1
94      "#{indent}#{line_counter}: #{line}\n"
95    end
96  end

but that does seem to be a valid use of the Array#sum method (https://ruby-doc.org/core-2.5.1/Array.html#method-i-sum). Is this something wrong with our codebase or is there an error in the ActionPack gem?