diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 53aa9e7..ba33879 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -1068,7 +1068,7 @@
 def register_namespace(prefix, uri):
     if re.match("ns\d+$", prefix):
         raise ValueError("Prefix format reserved for internal use")
-    for k, v in _namespace_map.items():
+    for k, v in list(_namespace_map.items()):
         if k == uri or v == prefix:
             del _namespace_map[k]
     _namespace_map[uri] = prefix