diff --git a/Lib/subprocess.py b/Lib/subprocess.py
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -1317,7 +1317,10 @@
             elif stderr == PIPE:
                 errread, errwrite = os.pipe()
             elif stderr == STDOUT:
-                errwrite = c2pwrite
+                if c2pwrite != -1:
+                    errwrite = c2pwrite
+                else: # child's stdout is not set, use parent's stdout
+                    errwrite = sys.__stdout__.fileno()
             elif stderr == DEVNULL:
                 errwrite = self._get_devnull()
             elif isinstance(stderr, int):
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -517,6 +517,27 @@
         tf.seek(0)
         self.assertStderrEqual(tf.read(), b"strawberry")
 
+    def test_stderr_redirect_with_no_stdout_redirect(self):
+        # test stderr=STDOUT while stdout=None (not set)
+
+        # - grandchild prints to stderr
+        # - child redirects grandchild's stderr to its stdout
+        # - the parent should get grandchild's stderr in child's stdout
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import sys, subprocess;'
+                              'rc = subprocess.call([sys.executable, "-c",'
+                              '    "import sys;"'
+                              '    "print(42, end=\'\', file=sys.stderr)"],'
+                              '    stderr=subprocess.STDOUT);'
+                              'sys.exit(rc)'],
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE)
+        stdout, stderr = p.communicate()
+        #NOTE: stdout should get stderr from grandchild
+        self.assertStderrEqual(stdout, b'42')
+        self.assertStderrEqual(stderr, b'') # should be empty
+        self.assertEqual(p.returncode, 0)
+
     def test_stdout_stderr_pipe(self):
         # capture stdout and stderr to the same pipe
         p = subprocess.Popen([sys.executable, "-c",