diff -r 22c8e6d71529 Lib/test/test_complex.py
--- a/Lib/test/test_complex.py	Sun Oct 28 10:23:08 2012 +0000
+++ b/Lib/test/test_complex.py	Sun Oct 28 11:44:44 2012 +0000
@@ -221,6 +221,8 @@
         self.assertRaises(TypeError, complex, OS(None))
         self.assertRaises(TypeError, complex, NS(None))
         self.assertRaises(TypeError, complex, {})
+        self.assertRaises(TypeError, complex, NS(1.5))
+        self.assertRaises(TypeError, complex, NS(1))
 
         self.assertAlmostEqual(complex("1+10j"), 1+10j)
         self.assertAlmostEqual(complex(10), 10+0j)
diff -r 22c8e6d71529 Misc/NEWS
--- a/Misc/NEWS	Sun Oct 28 10:23:08 2012 +0000
+++ b/Misc/NEWS	Sun Oct 28 11:44:44 2012 +0000
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #16290: A float return value from the __complex__ special method is no
+  longer accepted in the complex() constructor.
+
 - Issue #16241: Document -X faulthandler command line option.
   Patch by Marek Šuppa.
 
diff -r 22c8e6d71529 Objects/complexobject.c
--- a/Objects/complexobject.c	Sun Oct 28 10:23:08 2012 +0000
+++ b/Objects/complexobject.c	Sun Oct 28 11:44:44 2012 +0000
@@ -271,6 +271,12 @@
     if (f) {
         PyObject *res = PyObject_CallFunctionObjArgs(f, NULL);
         Py_DECREF(f);
+        if (res != NULL && !PyComplex_Check(res)) {
+            PyErr_SetString(PyExc_TypeError,
+                "__complex__ should return a complex object");
+            Py_DECREF(res);
+            return NULL;
+        }
         return res;
     }
     return NULL;
@@ -296,12 +302,6 @@
     newop = try_complex_special_method(op);
 
     if (newop) {
-        if (!PyComplex_Check(newop)) {
-            PyErr_SetString(PyExc_TypeError,
-                "__complex__ should return a complex object");
-            Py_DECREF(newop);
-            return cv;
-        }
         cv = ((PyComplexObject *)newop)->cval;
         Py_DECREF(newop);
         return cv;