NoMethodError: undefined method `map' for "15":String

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2991109/?referrer=gitlab_plugin

NoMethodError: undefined method `map' for "15":String
Did you mean?  tap
  lib/gitlab/diff/file.rb:459:in `preprocess_before_diff'
    transformed_diff = IpynbDiff.diff(from, to,
  lib/gitlab/diff/file.rb:47:in `initialize'
    preprocess_before_diff(diff) if Feature.enabled?(:jupyter_clean_diffs, repository.project, default_enabled: true)
  lib/gitlab/diff/file_collection/base.rb:123:in `new'
    Gitlab::Diff::File.new(diff,
  lib/gitlab/diff/file_collection/base.rb:123:in `decorate_diff!'
    Gitlab::Diff::File.new(diff,
  lib/gitlab/diff/file_collection/base.rb:40:in `block (2 levels) in raw_diff_files'
    collection = diffs.decorate! { |diff| decorate_diff!(diff) }
...
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