diff -r c61d2c43d22e Lib/test/test_decimal.py
--- a/Lib/test/test_decimal.py	Sun May 08 14:02:35 2016 +0000
+++ b/Lib/test/test_decimal.py	Sun May 08 18:24:23 2016 +0300
@@ -2548,6 +2548,19 @@ class PythonAPItests(unittest.TestCase):
             x = random.expovariate(0.01) * (random.random() * 2.0 - 1.0)
             self.assertEqual(x, float(MyDecimal.from_float(x))) # roundtrip
 
+        # Issue #26974: Crash when as_integer_ratio() returns insane value.
+        class BadFloat(float):
+            def as_integer_ratio(self):
+                return ratio
+            def __abs__(self):
+                return self
+        ratio = 1
+        self.assertRaises(TypeError, Decimal.from_float, BadFloat(1.2))
+        ratio = 1,
+        self.assertRaises(ValueError, Decimal.from_float, BadFloat(1.2))
+        ratio = 1, 2, 3
+        self.assertRaises(ValueError, Decimal.from_float, BadFloat(1.2))
+
     def test_create_decimal_from_float(self):
         Decimal = self.decimal.Decimal
         Context = self.decimal.Context
diff -r c61d2c43d22e Modules/_decimal/_decimal.c
--- a/Modules/_decimal/_decimal.c	Sun May 08 14:02:35 2016 +0000
+++ b/Modules/_decimal/_decimal.c	Sun May 08 18:24:23 2016 +0300
@@ -2268,6 +2268,18 @@ PyDecType_FromFloatExact(PyTypeObject *t
     if (n_d == NULL) {
         return NULL;
     }
+    if (!PyTuple_Check(n_d)) {
+        Py_DECREF(n_d);
+        PyErr_SetString(PyExc_TypeError,
+                        "as_integer_ratio() returned wrong type");
+        return NULL;
+    }
+    if (PyTuple_GET_SIZE(n_d) != 2) {
+        Py_DECREF(n_d);
+        PyErr_SetString(PyExc_ValueError,
+                    "as_integer_ratio() returned a tuple with wrong size");
+        return NULL;
+    }
     n = PyTuple_GET_ITEM(n_d, 0);
     d = PyTuple_GET_ITEM(n_d, 1);