diff --git a/lib/ulid/generate.rb b/lib/ulid/generate.rb
index 10e6dbf..efcf1ef 100644
--- a/lib/ulid/generate.rb
+++ b/lib/ulid/generate.rb
@@ -14,7 +14,7 @@ def encode32
 
       id = @bytes.bytes
 
-      output = +''
+      output = String.new('', encoding: Encoding::UTF_8)
 
       # Base32 encodes 5 bits of each byte of the input in each byte of the
       # output. That means each input byte produces >1 output byte and some