diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 2e8cd6d7d52..780955266ed 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -114,7 +114,7 @@ class _RandomNameSequence:
 
     characters = ("abcdefghijklmnopqrstuvwxyz" +
                   "ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
-                  "0123456789-_")
+                  "0123456789_")
 
     def __init__(self):
         self.mutex = _allocate_lock()