use the stricter PyMapping_Check (closes #15801)
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 5931f3d..a161512 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -1113,6 +1113,9 @@
         self.checkraises(TypeError, '%10.*f', '__mod__', ('foo', 42.))
         self.checkraises(ValueError, '%10', '__mod__', (42,))
 
+        class X(object): pass
+        self.checkraises(TypeError, 'abc', '__mod__', X())
+
     def test_floatformatting(self):
         # float formatting
         for prec in xrange(100):
diff --git a/Misc/NEWS b/Misc/NEWS
index 71a858e..54c260c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -9,6 +9,9 @@
 Core and Builtins
 -----------------
 
+- Issue #15801: Make sure mappings passed to '%' formatting are actually
+  subscriptable.
+
 - Issue #15604: Update uses of PyObject_IsTrue() to check for and handle
   errors correctly.  Patch by Serhiy Storchaka.
 
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index fd2f630..7c4a86b 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -4254,7 +4254,7 @@
         arglen = -1;
         argidx = -2;
     }
-    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
+    if (PyMapping_Check(args) && !PyTuple_Check(args) &&
         !PyObject_TypeCheck(args, &PyBaseString_Type))
         dict = args;
     while (--fmtcnt >= 0) {
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 59b138a..d40f2e4 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -8275,7 +8275,7 @@
         arglen = -1;
         argidx = -2;
     }
-    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
+    if (PyMapping_Check(args) && !PyTuple_Check(args) &&
         !PyObject_TypeCheck(args, &PyBaseString_Type))
         dict = args;