diff --git a/Lib/subprocess.py b/Lib/subprocess.py
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -800,7 +800,10 @@
         self.wait()
 
     def __del__(self, _maxsize=sys.maxsize, _active=_active):
-        if not self._child_created:
+        # If __init__ hasn't had a chance to execute (e.g. if it
+        # was passed an undeclared keyword argument), we don't
+        # have a _child_created attribute at all.
+        if not getattr(self, '_child_created', False):
             # We didn't get to successfully create a child process.
             return
         # In case the child hasn't been waited on, check if it's done.
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
@@ -146,6 +146,16 @@
                              env=newenv)
         self.assertEqual(rc, 1)
 
+    def test_invalid_args(self):
+        # Popen() called with invalid arguments should raise TypeError
+        # but Popen.__del__ should not complain (issue #12085)
+        with support.captured_stderr() as s:
+            self.assertRaises(TypeError, subprocess.Popen, invalid_arg_name=1)
+            argcount = subprocess.Popen.__init__.__code__.co_argcount
+            too_many_args = [0] * (argcount + 1)
+            self.assertRaises(TypeError, subprocess.Popen, *too_many_args)
+        self.assertEqual(s.getvalue(), '')
+
     def test_stdin_none(self):
         # .stdin is None when not redirected
         p = subprocess.Popen([sys.executable, "-c", 'print("banana")'],