diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -5,6 +5,7 @@ import sys
 import unittest
 import pickle
 import weakref
+import errno
 
 from test.support import (TESTFN, unlink, run_unittest, captured_output,
                           gc_collect, cpython_only, no_tracing)
@@ -852,6 +853,13 @@ class ExceptionTests(unittest.TestCase):
             self.fail("RuntimeError not raised")
         self.assertEqual(wr(), None)
 
+    def test_errno_ENOTDIR(self):
+        # Issue #12802: "not a directory" errors are ENOTDIR even on Windows
+        with self.assertRaises(OSError) as cm:
+            os.listdir(__file__)
+        self.assertEqual(cm.exception.errno, errno.ENOTDIR, cm.exception)
+
+
 def test_main():
     run_unittest(ExceptionTests)
 
diff --git a/PC/errmap.h b/PC/errmap.h
--- a/PC/errmap.h
+++ b/PC/errmap.h
@@ -72,6 +72,7 @@ int winerror_to_errno(int winerror)
         case 202: return 8;
         case 206: return 2;
         case 215: return 11;
+        case 267: return 20;
         case 1816: return 12;
         default: return EINVAL;
     }
diff --git a/PC/generrmap.c b/PC/generrmap.c
--- a/PC/generrmap.c
+++ b/PC/generrmap.c
@@ -1,3 +1,6 @@
+#include <windows.h>
+#include <fcntl.h>
+#include <io.h>
 #include <stdio.h>
 #include <errno.h>
 
@@ -6,15 +9,21 @@
 int main()
 {
     int i;
+    _setmode(fileno(stdout), O_BINARY);
     printf("/* Generated file. Do not edit. */\n");
     printf("int winerror_to_errno(int winerror)\n");
-    printf("{\n\tswitch(winerror) {\n");
+    printf("{\n    switch(winerror) {\n");
     for(i=1; i < 65000; i++) {
         _dosmaperr(i);
-        if (errno == EINVAL)
-            continue;
-        printf("\t\tcase %d: return %d;\n", i, errno);
+        if (errno == EINVAL) {
+            /* Issue #12802 */
+            if (i == ERROR_DIRECTORY)
+                errno = ENOTDIR;
+            else
+                continue;
+        }
+        printf("        case %d: return %d;\n", i, errno);
     }
-    printf("\t\tdefault: return EINVAL;\n");
-    printf("\t}\n}\n");
+    printf("        default: return EINVAL;\n");
+    printf("    }\n}\n");
 }