diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 10ef87b..00dc37b 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -2742,6 +2742,15 @@
                           stdout=subprocess.PIPE,
                           close_fds=True)
 
+    @support.cpython_only
+    def test_issue31471(self):
+        # There shouldn't be an assertion failure in Popen() in case the env
+        # argument has a bad keys() method.
+        class BadEnv(dict):
+            keys = None
+        with self.assertRaises(TypeError):
+            subprocess.Popen([sys.executable, "-c", "pass"], env=BadEnv())
+
     def test_close_fds(self):
         # close file descriptors
         rc = subprocess.call([sys.executable, "-c",
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-14-19-47-57.bpo-31471.0yiA5Q.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-14-19-47-57.bpo-31471.0yiA5Q.rst
new file mode 100644
index 0000000..73c4444
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-14-19-47-57.bpo-31471.0yiA5Q.rst
@@ -0,0 +1,2 @@
+Fix an assertion failure in `subprocess.Popen()` on Windows, in case the env
+argument has a bad keys() method. Patch by Oren Milman.
diff --git a/Modules/_winapi.c b/Modules/_winapi.c
index a81ccaf..00a26d51 100644
--- a/Modules/_winapi.c
+++ b/Modules/_winapi.c
@@ -723,9 +723,13 @@
     }
 
     keys = PyMapping_Keys(environment);
+    if (!keys) {
+        return NULL;
+    }
     values = PyMapping_Values(environment);
-    if (!keys || !values)
+    if (!values) {
         goto error;
+    }
 
     envsize = PySequence_Fast_GET_SIZE(keys);
     if (PySequence_Fast_GET_SIZE(values) != envsize) {