diff -r 0a306e5ce5aa Lib/test/test_tcl.py
--- a/Lib/test/test_tcl.py	Sat Jan 04 22:49:40 2014 +0200
+++ b/Lib/test/test_tcl.py	Sun Jan 05 13:11:47 2014 +0200
@@ -171,6 +171,7 @@
         self.assertEqual(passValue(True), True if self.wantobjects else '1')
         self.assertEqual(passValue(False), False if self.wantobjects else '0')
         self.assertEqual(passValue('string'), 'string')
+        self.assertEqual(passValue('string\xbd'), 'string\xbd')
         self.assertEqual(passValue('string\u20ac'), 'string\u20ac')
         for i in (0, 1, -1, 2**31-1, -2**31):
             self.assertEqual(passValue(i), i if self.wantobjects else str(i))
@@ -194,6 +195,40 @@
         self.assertEqual(passValue((1, '2', (3.4,))),
                          (1, '2', (3.4,)) if self.wantobjects else '1 2 3.4')
 
+    def test_user_command(self):
+        result = None
+        def testfunc(arg):
+            nonlocal result
+            result = arg
+        self.interp.createcommand('testfunc', testfunc)
+
+        def check(value, expected):
+            self.interp.call('testfunc', value)
+            self.assertEqual(result, expected)
+
+        check(True, '1')
+        check(False, '0')
+        check('string', 'string')
+        check('string\xbd', 'string\xbd')
+        check('string\u20ac', 'string\u20ac')
+        check('string\udca2', 'string\ufffd\ufffd\ufffd')
+        check('string\ud801\udca2',
+                         'string\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd')
+        check('str\x00ing', 'str\x00ing')
+        check('str\x00ing\xbd', 'str\x00ing\xbd')
+        check('str\x00ing\u20ac', 'str\x00ing\u20ac')
+        for i in (0, 1, -1, 2**31-1, -2**31):
+            check(i, str(i))
+        for f in (0.0, 1.0, -1.0, 1/3,
+                  sys.float_info.min, sys.float_info.max,
+                  -sys.float_info.min, -sys.float_info.max):
+            check(f, str(f))
+        check(float('nan'), 'NaN')
+        check(float('inf'), 'Inf')
+        check(-float('inf'), '-Inf')
+        check((), '')
+        check((1, '2', (3.4,)), '1 2 3.4')
+
     def test_splitlist(self):
         splitlist = self.interp.tk.splitlist
         call = self.interp.tk.call
diff -r 0a306e5ce5aa Modules/_tkinter.c
--- a/Modules/_tkinter.c	Sat Jan 04 22:49:40 2014 +0200
+++ b/Modules/_tkinter.c	Sun Jan 05 13:11:47 2014 +0200
@@ -343,6 +343,44 @@
 
 
 static PyObject *
+fromTclStringAndSize(const char *s, Py_ssize_t size)
+{
+    PyObject *r = PyUnicode_DecodeUTF8(s, size, NULL);
+    if (!r && PyErr_ExceptionMatches(PyExc_UnicodeDecodeError)) {
+        char *buf = NULL;
+        PyErr_Clear();
+        /* Tcl encodes null character as \xc0\x80 */
+        if (memchr(s, '\xc0', size)) {
+            const char *e = s + size;
+            char *q = buf = (char *)PyMem_Malloc(size);
+            if (buf == NULL)
+                return NULL;
+            while (s != e) {
+                if (s + 1 != e && s[0] == '\xc0' && s[1] == '\x80') {
+                    *q++ = '\0';
+                    s += 2;
+                }
+                else
+                    *q++ = *s++;
+            }
+            s = buf;
+            size = q - s;
+        }
+        r = PyUnicode_DecodeUTF8(s, size, "replace");
+        if (buf != NULL)
+            PyMem_Free(buf);
+    }
+    return r;
+}
+
+static PyObject *
+fromTclString(const char *s)
+{
+    return fromTclStringAndSize(s, strlen(s));
+}
+
+
+static PyObject *
 Split(char *list)
 {
     int argc;
@@ -1899,20 +1937,8 @@
         return PythonCmd_Error(interp);
 
     for (i = 0; i < (argc - 1); i++) {
-        PyObject *s = PyUnicode_FromString(argv[i + 1]);
-        if (!s) {
-            /* Is Tk leaking 0xC080 in %A - a "modified" utf-8 null? */
-            if (PyErr_ExceptionMatches(PyExc_UnicodeDecodeError) &&
-                !strcmp(argv[i + 1], "\xC0\x80")) {
-                PyErr_Clear();
-                /* Convert to "strict" utf-8 null */
-                s = PyUnicode_FromString("\0");
-            } else {
-                Py_DECREF(arg);
-                return PythonCmd_Error(interp);
-            }
-        }
-        if (PyTuple_SetItem(arg, i, s)) {
+        PyObject *s = fromTclString(argv[i + 1]);
+        if (!s || PyTuple_SetItem(arg, i, s)) {
             Py_DECREF(arg);
             return PythonCmd_Error(interp);
         }