diff --git a/Misc/NEWS b/Misc/NEWS
index b846d8fb54f..8594c99e6f5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@ What's New in Python 3.0 release candidate 1
 Core and Builtins
 -----------------
 
+- Issue #3657: Fix uninitialized memory read when pickling longs.
+  Found by valgrind.
+
 - Apply security patches from Apple.
 
 - Fix crashes on memory allocation failure found with failmalloc.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 52fa15694cd..ea5bbe2759a 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -924,10 +924,10 @@ save_long(PicklerObject *self, PyObject *obj)
                             "long too large to pickle");
             goto error;
         }
-        repr = PyUnicode_FromStringAndSize(NULL, (int)nbytes);
+        repr = PyBytes_FromStringAndSize(NULL, (Py_ssize_t)nbytes);
         if (repr == NULL)
             goto error;
-        pdata = (unsigned char *)_PyUnicode_AsString(repr);
+        pdata = (unsigned char *)PyBytes_AS_STRING(repr);
         i = _PyLong_AsByteArray((PyLongObject *)obj,
                                 pdata, nbytes,
                                 1 /* little endian */ , 1 /* signed */ );