diff --git a/lib/psych/nodes/node.rb b/lib/psych/nodes/node.rb
index 1a4ea553..6ae5c591 100644
--- a/lib/psych/nodes/node.rb
+++ b/lib/psych/nodes/node.rb
@@ -55,7 +55,8 @@ def to_ruby(symbolize_names: false, freeze: false, strict_integer: false)
       #
       # See also Psych::Visitors::Emitter
       def yaml io = nil, options = {}
-        require "stringio"
+        require "stringio" unless defined?(StringIO)
+
         real_io = io || StringIO.new(''.encode('utf-8'))
 
         Visitors::Emitter.new(real_io, options).accept self