diff --git a/Objects/longobject.c b/Objects/longobject.c
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -1246,8 +1246,7 @@
 PyLong_AsUnsignedLongLong(PyObject *vv)
 {
     PyLongObject *v;
-    unsigned PY_LONG_LONG bytes;
-    int res;
+    unsigned PY_LONG_LONG result = 0;
 
     if (vv == NULL) {
         PyErr_BadInternalCall();
@@ -1260,18 +1259,28 @@
 
     v = (PyLongObject*)vv;
     switch(Py_SIZE(v)) {
-    case 0: return 0;
-    case 1: return v->ob_digit[0];
-    }
-
-    res = _PyLong_AsByteArray((PyLongObject *)vv, (unsigned char *)&bytes,
-                              SIZEOF_LONG_LONG, PY_LITTLE_ENDIAN, 0);
-
-    /* Plan 9 can't handle PY_LONG_LONG in ? : expressions */
-    if (res < 0)
-        return (unsigned PY_LONG_LONG)res;
-    else
-        return bytes;
+#define DIGIT_AS_LLU(n) ((unsigned PY_LONG_LONG)v->ob_digit[n] << (n*PyLong_SHIFT))
+#if PYLONG_BITS_IN_DIGIT == 15
+    case 5: if (v->ob_digit[4] > 0xF) goto overflow_error;
+            result |= DIGIT_AS_LLU(4);
+    case 4: result |= DIGIT_AS_LLU(3);
+    case 3: result |= DIGIT_AS_LLU(2);
+#else
+    case 3: if (v->ob_digit[2] > 0xF) goto overflow_error;
+            result |= DIGIT_AS_LLU(2);
+#endif
+    case 2: result |= DIGIT_AS_LLU(1);
+    case 1: result |= DIGIT_AS_LLU(0);
+    case 0: break;
+    default: goto overflow_error;
+    }
+
+    return result;
+
+overflow_error:
+    PyErr_Format(PyExc_OverflowError,
+                 "input out of range for unsigned long long: %R", vv);
+    return (unsigned PY_LONG_LONG)-1;
 }
 
 /* Get a C unsigned long int from an int object, ignoring the high bits.