diff --git a/Library/Homebrew/utils.rb b/Library/Homebrew/utils.rb
index d87b835628..6eb651979d 100644
--- a/Library/Homebrew/utils.rb
+++ b/Library/Homebrew/utils.rb
@@ -143,11 +143,11 @@ module Utils
   #   `ActiveSupport::Inflector.underscore`
   sig { params(camel_cased_word: T.any(String, Symbol)).returns(String) }
   def self.underscore(camel_cased_word)
-    return camel_cased_word unless /[A-Z-]|::/.match?(camel_cased_word)
+    return camel_cased_word.to_s unless /[A-Z-]|::/.match?(camel_cased_word)
 
     word = camel_cased_word.to_s.gsub("::", "/")
     word.gsub!(/([A-Z])(?=[A-Z][a-z])|([a-z\d])(?=[A-Z])/) do
-      (::Regexp.last_match(1) || ::Regexp.last_match(2)) << "_"
+      T.must(::Regexp.last_match(1) || ::Regexp.last_match(2)) << "_"
     end
     word.tr!("-", "_")
     word.downcase!