diff --git a/ext/openssl/extconf.rb b/ext/openssl/extconf.rb
index 467f7e7a86..26abfc8b2a 100644
--- a/ext/openssl/extconf.rb
+++ b/ext/openssl/extconf.rb
@@ -185,7 +185,7 @@ have_func("EVP_PKEY_dup")
 Logging::message "=== Checking done. ===\n"
 
 if openssl_3
-  if $warnflags.sub!(/-W\K(?=deprecated-declarations)/, 'no-')
+  if $warnflags&.sub!(/-W\K(?=deprecated-declarations)/, 'no-')
     $warnflags << " -Wno-incompatible-pointer-types-discards-qualifiers"
   end
 end