diff -r 6ff9eef4e7c3 Lib/test/test_cmath.py
--- a/Lib/test/test_cmath.py	Sat Oct 20 11:45:23 2012 +1000
+++ b/Lib/test/test_cmath.py	Sat Oct 20 10:50:03 2012 +0100
@@ -165,7 +165,7 @@
 
         # a variety of non-complex numbers, used to check that
         # non-complex return values from __complex__ give an error
-        non_complexes = ["not complex", 1, 5, 2., None,
+        non_complexes = ["not complex", 1, 5, None,
                          object(), NotImplemented]
 
         # Now we introduce a variety of classes whose instances might
@@ -235,6 +235,8 @@
             self.assertEqual(f(FloatAndComplexOS()), f(cx_arg))
             self.assertEqual(f(JustFloat()), f(flt_arg))
             self.assertEqual(f(JustFloatOS()), f(flt_arg))
+            # __complex__ should be permitted to return a float (issue 16290).
+            self.assertEqual(f(MyComplex(flt_arg)), f(flt_arg))
             # TypeError should be raised for classes not providing
             # either __complex__ or __float__, even if they provide
             # __int__ or __index__.  An old-style class
diff -r 6ff9eef4e7c3 Objects/complexobject.c
--- a/Objects/complexobject.c	Sat Oct 20 11:45:23 2012 +1000
+++ b/Objects/complexobject.c	Sat Oct 20 10:50:03 2012 +0100
@@ -296,13 +296,17 @@
     newop = try_complex_special_method(op);
 
     if (newop) {
-        if (!PyComplex_Check(newop)) {
+        if (PyComplex_Check(newop)) {
+            cv = ((PyComplexObject *)newop)->cval;
+        }
+        else if (PyFloat_Check(newop)) {
+            cv.real = PyFloat_AS_DOUBLE((PyFloatObject *)newop);
+            cv.imag = 0.0;
+        }
+        else {
             PyErr_SetString(PyExc_TypeError,
                 "__complex__ should return a complex object");
-            Py_DECREF(newop);
-            return cv;
         }
-        cv = ((PyComplexObject *)newop)->cval;
         Py_DECREF(newop);
         return cv;
     }