diff --git a/lib/net/imap/command_data.rb b/lib/net/imap/command_data.rb
index 2022b21e..bd7299de 100644
--- a/lib/net/imap/command_data.rb
+++ b/lib/net/imap/command_data.rb
@@ -119,8 +119,9 @@ def non_sync_literal?(bytesize)
          bytesize <= 4096 && (capable?("IMAP4rev2") || capable?("LITERAL-")))
     end
 
+    # NOTE: +num+ should already be an Integer
     def send_number_data(num)
-      put_string(num.to_s)
+      put_string(Integer(num).to_s)
     end
 
     def send_list_data(list, tag = nil)