Index: Objects/unicodeobject.c
===================================================================
--- Objects/unicodeobject.c	(revision 73674)
+++ Objects/unicodeobject.c	(working copy)
@@ -4201,10 +4201,12 @@
                     repsize = PyBytes_Size(repunicode);
                     if (repsize > 1) {
                         /* Make room for all additional bytes. */
+                        respos = str - PyBytes_AS_STRING(res);
                         if (_PyBytes_Resize(&res, ressize+repsize-1)) {
                             Py_DECREF(repunicode);
                             goto onError;
                         }
+                        str = PyBytes_AS_STRING(res) + respos;
                         ressize += repsize-1;
                     }
                     memcpy(str, PyBytes_AsString(repunicode), repsize);
Index: Lib/test/test_codecs.py
===================================================================
--- Lib/test/test_codecs.py	(revision 73674)
+++ Lib/test/test_codecs.py	(working copy)
@@ -1549,7 +1549,12 @@
         self.assertEqual("foo\udca5bar".encode("iso-8859-3", "surrogateescape"),
                          b"foo\xa5bar")
 
+    def test_latin1(self):
+        # Issue6373
+        self.assertEqual("\udce4\udceb\udcef\udcf6\udcfc".encode("latin1", "surrogateescape"),
+                         b"\xe4\xeb\xef\xf6\xfc")
 
+
 def test_main():
     support.run_unittest(
         UTF32Test,