#10777: fix iteration over dict keys while mutating the dict.
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