Summary

ActiveSupport::Inflector#parameterize contains a guard that explicitly handles a nil separator:

unless separator.nil? || separator.empty?
  # No more than one of the separator in a row.
  # Remove leading/trailing separator.
end

But three lines earlier the separator is passed straight to String#gsub!:

parameterized_string.gsub!(/[^a-z0-9\-_]+/i, separator)

so a nil separator raises before the guard is ever reached, and the separator.nil? branch is unreachable:

"Donald E. Knuth".parameterize(separator: nil)
# => TypeError: no implicit conversion of nil into String

Fix

Treat a nil separator like an empty one — which is exactly what the existing guard already implies — and drop the now-redundant separator.nil? check:

"Donald E. Knuth".parameterize(separator: nil) # => "donaldeknuth"
"Donald E. Knuth".parameterize(separator: "")  # => "donaldeknuth"

The alternative reading — that nil was never meant to be supported — would mean deleting the separator.nil? branch instead. Making it work seemed like the smaller surprise for callers that pass a separator through from configuration or params, but happy to flip it to the removal if maintainers prefer that.

Both separator: nil and separator: "" now go down the same path, so no behavior changes for any currently-working input.

Tests

Added separator: nil cases mirroring the existing separator: "" tests, including the preserve_case: true variant, reusing the existing StringToParameterizeWithNoSeparator and StringToParameterizePreserveCaseWithNoSeparator fixtures.

activesupport — string_ext_test.rb, inflector_test.rb, transliterate_test.rb: 645 runs, 3207 assertions, 0 failures, 0 errors. RuboCop clean.