diff -r cca2ed4e8b41 Include/genobject.h
--- a/Include/genobject.h	Thu Dec 17 10:35:05 2015 +0000
+++ b/Include/genobject.h	Thu Dec 17 16:21:15 2015 -0500
@@ -43,6 +43,7 @@
 PyAPI_FUNC(int) PyGen_NeedsFinalizing(PyGenObject *);
 PyAPI_FUNC(int) _PyGen_FetchStopIterationValue(PyObject **);
 PyObject *_PyGen_Send(PyGenObject *, PyObject *);
+PyObject *_PyGen_yf(PyGenObject *);
 PyAPI_FUNC(void) _PyGen_Finalize(PyObject *self);
 
 #ifndef Py_LIMITED_API
diff -r cca2ed4e8b41 Lib/test/test_coroutines.py
--- a/Lib/test/test_coroutines.py	Thu Dec 17 10:35:05 2015 +0000
+++ b/Lib/test/test_coroutines.py	Thu Dec 17 16:21:15 2015 -0500
@@ -801,6 +801,24 @@
         with self.assertRaises(Marker):
             c.throw(ZeroDivisionError)
 
+    def test_await_15(self):
+        @types.coroutine
+        def nop():
+            yield
+
+        async def coroutine():
+            await nop()
+
+        async def waiter(coro):
+            await coro
+
+        coro = coroutine()
+        coro.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    "coroutine is being awaited already"):
+            waiter(coro).send(None)
+
     def test_with_1(self):
         class Manager:
             def __init__(self, name):
diff -r cca2ed4e8b41 Objects/genobject.c
--- a/Objects/genobject.c	Thu Dec 17 10:35:05 2015 +0000
+++ b/Objects/genobject.c	Thu Dec 17 16:21:15 2015 -0500
@@ -258,8 +258,8 @@
     return 0;
 }
 
-static PyObject *
-gen_yf(PyGenObject *gen)
+PyObject *
+_PyGen_yf(PyGenObject *gen)
 {
     PyObject *yf = NULL;
     PyFrameObject *f = gen->gi_frame;
@@ -281,7 +281,7 @@
 gen_close(PyGenObject *gen, PyObject *args)
 {
     PyObject *retval;
-    PyObject *yf = gen_yf(gen);
+    PyObject *yf = _PyGen_yf(gen);
     int err = 0;
 
     if (yf) {
@@ -321,7 +321,7 @@
     PyObject *typ;
     PyObject *tb = NULL;
     PyObject *val = NULL;
-    PyObject *yf = gen_yf(gen);
+    PyObject *yf = _PyGen_yf(gen);
     _Py_IDENTIFIER(throw);
 
     if (!PyArg_UnpackTuple(args, "throw", 1, 3, &typ, &val, &tb))
@@ -555,7 +555,7 @@
 static PyObject *
 gen_getyieldfrom(PyGenObject *gen)
 {
-    PyObject *yf = gen_yf(gen);
+    PyObject *yf = _PyGen_yf(gen);
     if (yf == NULL)
         Py_RETURN_NONE;
     return yf;
@@ -790,7 +790,7 @@
 static PyObject *
 coro_get_cr_await(PyCoroObject *coro)
 {
-    PyObject *yf = gen_yf((PyGenObject *) coro);
+    PyObject *yf = _PyGen_yf((PyGenObject *) coro);
     if (yf == NULL)
         Py_RETURN_NONE;
     return yf;
diff -r cca2ed4e8b41 Python/ceval.c
--- a/Python/ceval.c	Thu Dec 17 10:35:05 2015 +0000
+++ b/Python/ceval.c	Thu Dec 17 16:21:15 2015 -0500
@@ -2021,6 +2021,21 @@
 
             Py_DECREF(iterable);
 
+            if (iter != NULL && PyCoro_CheckExact(iter)) {
+                PyObject *yf = _PyGen_yf((PyGenObject*)iter);
+                if (yf != NULL) {
+                    /* `iter` is a coroutine object that is being
+                       awaited, `yf` is a pointer the current awaitable
+                       being awaited on. */
+                    Py_DECREF(yf);
+                    Py_CLEAR(iter);
+                    PyErr_SetString(
+                        PyExc_RuntimeError,
+                        "coroutine is being awaited already");
+                    /* The code below jumps to `error` if `iter` is NULL. */
+                }
+            }
+
             SET_TOP(iter); /* Even if it's NULL */
 
             if (iter == NULL) {