diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
index 607d9f9..0023363 100644
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -584,6 +584,17 @@
         self.assertNotIn(b'Warning!', stderr)
         self.assertNotIn(b'Error', stderr)
 
+    @test_support.cpython_only
+    def test_issue31411(self):
+        # warn_explicit() shouldn't raise a SystemError in case
+        # warnings.onceregistry isn't a dictionary.
+        wmod = self.module
+        with original_warnings.catch_warnings(module=wmod):
+            wmod.filterwarnings('once')
+            with test_support.swap_attr(wmod, 'onceregistry', None):
+                with self.assertRaises(TypeError):
+                    wmod.warn_explicit('foo', Warning, 'bar', 1, registry=None)
+
 
 class WarningsDisplayTests(unittest.TestCase):
 
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-11-08-50-41.bpo-31411.HZz82I.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-11-08-50-41.bpo-31411.HZz82I.rst
new file mode 100644
index 0000000..ad1b4b8
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-11-08-50-41.bpo-31411.HZz82I.rst
@@ -0,0 +1,2 @@
+Raise a TypeError instead of SystemError in case warnings.onceregistry is
+not a dictionary. Patch by Oren Milman.
diff --git a/Python/_warnings.c b/Python/_warnings.c
index 8e8c0cc..dd168f9 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -72,6 +72,12 @@
             return NULL;
         return _once_registry;
     }
+    if (!PyDict_Check(registry)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "warnings.onceregistry must be a dict");
+        Py_DECREF(registry);
+        return NULL;
+    }
     Py_DECREF(_once_registry);
     _once_registry = registry;
     return registry;
@@ -296,7 +302,7 @@
     int rc;
 
     if (registry && !PyDict_Check(registry) && (registry != Py_None)) {
-        PyErr_SetString(PyExc_TypeError, "'registry' must be a dict");
+        PyErr_SetString(PyExc_TypeError, "'registry' must be a dict or None");
         return NULL;
     }