diff --git a/mercurial/windows.py b/mercurial/windows.py
--- a/mercurial/windows.py
+++ b/mercurial/windows.py
@@ -581,7 +581,13 @@ def username(uid=None):
 
     If uid is None, return the name of the current user."""
     if not uid:
-        return pycompat.fsencode(getpass.getuser())
+        try:
+            return pycompat.fsencode(getpass.getuser())
+        except ModuleNotFoundError:
+            # getpass.getuser() checks for a few environment variables first,
+            # but if those aren't set, imports pwd and calls getpwuid(), none of
+            # which exists on Windows.
+            pass
     return None
 
 
diff --git a/tests/test-issue1102.t b/tests/test-issue1102.t
--- a/tests/test-issue1102.t
+++ b/tests/test-issue1102.t
@@ -14,4 +14,18 @@
   tip                                3:a49829c4fc11
   t1                                 0:f7b1eb17ad24
 
+Ensure that the username access fails gracefully if assumptions about the
+environment made by python do not hold.
+
+#if windows
+  >>> import os
+  >>> from mercurial import util
+  >>> os.environ.pop('LOGNAME', None) and None
+  >>> os.environ.pop('USER', None) and None
+  >>> os.environ.pop('LNAME', None) and None
+  >>> os.environ.pop('USERNAME', None) and None
+  >>> print(util.username())
+  None
+#endif
+
   $ cd ..