class RuboCop::Cop::RSpec::ContextWording

def prefixes
  Array(cop_config.fetch('Prefixes', [])).tap do |prefixes|
    non_strings = prefixes.reject { |pre| pre.is_a?(String) }
    unless non_strings.empty?
      raise "Non-string prefixes #{non_strings.inspect} detected."
    end
  end
end