diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -3278,6 +3278,11 @@
                         self.assertEqual(c4.prec, 4)
                         del c4
 
+    def test_localcontext_none(self):
+        # see #15783
+        with self.decimal.localcontext(None):
+            pass
+
 class CContextWithStatement(ContextWithStatement):
     decimal = C
 class PyContextWithStatement(ContextWithStatement):
diff --git a/Modules/_decimal/_decimal.c b/Modules/_decimal/_decimal.c
--- a/Modules/_decimal/_decimal.c
+++ b/Modules/_decimal/_decimal.c
@@ -1486,10 +1486,10 @@
     }
 
 #define CONTEXT_CHECK_VA(obj) \
-    if (!PyDecContext_Check(obj)) {                 \
-        PyErr_SetString(PyExc_TypeError,            \
-            "optional argument must be a context"); \
-        return NULL;                                \
+    if ((obj != Py_None) && !PyDecContext_Check(obj)) { \
+        PyErr_SetString(PyExc_TypeError,                \
+            "optional argument must be a context");     \
+        return NULL;                                    \
     }