diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 8da3e77..e4688d0 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -1206,6 +1206,9 @@
         self.checkraises(ValueError, '%%%df' % (2**64), '__mod__', (3.2))
         self.checkraises(ValueError, '%%.%df' % (2**64), '__mod__', (3.2))
 
+        class X(object): pass
+        self.checkraises(TypeError, 'abc', '__mod__', X())
+
     def test_floatformatting(self):
         # float formatting
         for prec in range(100):
diff --git a/Misc/NEWS b/Misc/NEWS
index 17d0c10..2d46263 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -74,6 +74,9 @@
 
 - Issue #15761: Fix crash when PYTHONEXECUTABLE is set on Mac OS X.
 
+- Issue #15801: Make sure mappings passed to '%' formatting are actually
+  subscriptable.
+
 - Issue #15726: Fix incorrect bounds checking in PyState_FindModule.
   Patch by Robin Schreiber.
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 773a9be..af4a13a 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -13461,8 +13461,7 @@
         arglen = -1;
         argidx = -2;
     }
-    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
-        !PyUnicode_Check(args))
+    if (PyMapping_Check(args) && !PyTuple_Check(args) && !PyUnicode_Check(args))
         dict = args;
 
     while (--fmtcnt >= 0) {