diff --git a/mercurial/encoding.py b/mercurial/encoding.py
--- a/mercurial/encoding.py
+++ b/mercurial/encoding.py
@@ -290,6 +290,42 @@
         for k, v in os.environ.items()  # re-exports
     }
 
+# Environment variables are normalized to uppercase on Windows. So wrap
+# in a custom type to emulate what the standard library does.
+if pycompat.ispy3 and pycompat.iswindows:
+
+    class UppercaseDict(dict):
+        def __init__(self, values):
+            super(UppercaseDict, self).__init__()
+            for k, v in values.items():
+                self[k] = v
+
+        def __setitem__(self, key, value):
+            super(UppercaseDict, self).__setitem__(key.upper(), value)
+
+        def __getitem__(self, key):
+            return super(UppercaseDict, self).__getitem__(key.upper())
+
+        def __delitem__(self, key):
+            super(UppercaseDict, self).__delitem__(key.upper())
+
+        def __contains__(self, key):
+            return super(UppercaseDict, self).__contains__(key.upper())
+
+        def get(self, key, *args):
+            return super(UppercaseDict, self).get(key.upper(), *args)
+
+        def pop(self, key, *args):
+            return super(UppercaseDict, self).pop(key.upper(), *args)
+
+        def update(self):
+            raise NotImplementedError
+
+        def setdefault(self, key, *args):
+            return super(UppercaseDict, self).setdefault(key.upper(), *args)
+
+    environ = UppercaseDict(environ)
+
 if pycompat.ispy3:
     # os.getcwd() on Python 3 returns string, but it has os.getcwdb() which
     # returns bytes.