diff --git a/lib/nokogiri/xml/node.rb b/lib/nokogiri/xml/node.rb
index 494e76abc6..604b3e46e7 100644
--- a/lib/nokogiri/xml/node.rb
+++ b/lib/nokogiri/xml/node.rb
@@ -1191,7 +1191,7 @@ def serialize(*args, &block)
         options[:encoding] ||= document.encoding
         encoding = Encoding.find(options[:encoding] || "UTF-8")
 
-        io = StringIO.new(String.new(encoding: encoding), "wb:#{encoding}:#{encoding}")
+        io = StringIO.new(String.new(encoding: encoding))
 
         write_to(io, options, &block)
         io.string
diff --git a/test/xml/test_document_encoding.rb b/test/xml/test_document_encoding.rb
index e4b3f89a6b..67b5cf05ae 100644
--- a/test/xml/test_document_encoding.rb
+++ b/test/xml/test_document_encoding.rb
@@ -50,7 +50,7 @@ class TestDocumentEncoding < Nokogiri::TestCase
 
           # the document needs to be large enough to trigger a libxml2 buffer flush. the buffer size
           # is determined by MINLEN in xmlIO.c, which is hardcoded to 4000 code points.
-          size = 4000
+          size = 8000
           input = String.new(<<~XML, encoding: "UTF-16")
             <?xml version="1.0" encoding="UTF-16"?>
             <root>