diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 340470029..f28c99e67 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -1237,7 +1237,7 @@ def send_quoted_string(str)
     end
 
     def send_literal(str)
-      put_string("{" + str.length.to_s + "}" + CRLF)
+      put_string("{" + str.bytesize.to_s + "}" + CRLF)
       @continuation_request_arrival.wait
       raise @exception if @exception
       put_string(str)
@@ -2571,7 +2571,7 @@ def format_string(str)
           return '""'
         when /[\x80-\xff\r\n]/n
           # literal
-          return "{" + str.length.to_s + "}" + CRLF + str
+          return "{" + str.bytesize.to_s + "}" + CRLF + str
         when /[(){ \x00-\x1f\x7f%*"\\]/n
           # quoted string
           return '"' + str.gsub(/["\\]/n, "\\\\\\&") + '"'