diff --git a/Lib/lib-tk/test/test_tkinter/test_variables.py b/Lib/lib-tk/test/test_tkinter/test_variables.py
index fe64ed2..a24ea38 100644
--- a/Lib/lib-tk/test/test_tkinter/test_variables.py
+++ b/Lib/lib-tk/test/test_tkinter/test_variables.py
@@ -58,6 +58,14 @@
         with self.assertRaises(TypeError):
             Variable(self.root, name=123)
 
+    def test_null_in_name(self):
+        with self.assertRaises(ValueError):
+            Variable(self.root, name='var\x00name')
+        with self.assertRaises(ValueError):
+            self.root.globalsetvar('var\x00name', "value")
+        with self.assertRaises(ValueError):
+            self.root.setvar('var\x00name', "value")
+
 
 class TestStringVar(TestBase):
 
@@ -71,6 +79,12 @@
         self.root.globalsetvar("name", "value")
         self.assertEqual("value", v.get())
 
+    def test_get_null(self):
+        v = StringVar(self.root, "abc\x00def", "name")
+        self.assertEqual("abc\x00def", v.get())
+        self.root.globalsetvar("name", "val\x00ue")
+        self.assertEqual("val\x00ue", v.get())
+
 
 class TestIntVar(TestBase):
 
diff --git a/Lib/test/test_tcl.py b/Lib/test/test_tcl.py
index 8f262d0..2dad5a0 100644
--- a/Lib/test/test_tcl.py
+++ b/Lib/test/test_tcl.py
@@ -139,6 +139,18 @@
         self.assertEqual(tcl.eval('set b'),'2')
         self.assertEqual(tcl.eval('set c'),'3')
 
+    def test_evalfile_null_in_result(self):
+        tcl = self.interp
+        with open(test_support.TESTFN, 'wb') as f:
+            self.addCleanup(test_support.unlink, test_support.TESTFN)
+            f.write("""
+            set a "a\0b"
+            set b "a\\0b"
+            """)
+        tcl.evalfile(test_support.TESTFN)
+        self.assertEqual(tcl.eval('set a'), 'a\xc0\x80b')
+        self.assertEqual(tcl.eval('set b'), 'a\xc0\x80b')
+
     def testEvalFileException(self):
         tcl = self.interp
         filename = "doesnotexists"
@@ -220,6 +232,7 @@
         check('"abc"', 'abc')
         check('"a\xc2\xbd\xe2\x82\xac"', 'a\xc2\xbd\xe2\x82\xac')
         check(r'"a\xbd\u20ac"', 'a\xc2\xbd\xe2\x82\xac')
+        check(r'"a\0b"', 'a\xc0\x80b')
 
     def test_exprdouble(self):
         tcl = self.interp
@@ -326,8 +339,17 @@
 
         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\xe2\x82\xac'), u'string\u20ac')
         self.assertEqual(passValue(u'string'), u'string')
+        self.assertEqual(passValue(u'string\xbd'), u'string\xbd')
         self.assertEqual(passValue(u'string\u20ac'), u'string\u20ac')
+        self.assertEqual(passValue('str\x00ing'), 'str\x00ing')
+        self.assertEqual(passValue('str\xc0\x80ing'), 'str\x00ing')
+        self.assertEqual(passValue(u'str\x00ing'), u'str\x00ing')
+        self.assertEqual(passValue(u'str\x00ing\xbd'), u'str\x00ing\xbd')
+        self.assertEqual(passValue(u'str\x00ing\u20ac'), u'str\x00ing\u20ac')
         for i in (0, 1, -1, int(2**31-1), int(-2**31)):
             self.assertEqual(passValue(i), i if self.wantobjects else str(i))
         for f in (0.0, 1.0, -1.0, 1//3, 1/3.0,
@@ -356,14 +378,16 @@
             result.append(arg)
             return arg
         self.interp.createcommand('testfunc', testfunc)
-        def check(value, expected, eq=self.assertEqual):
+        def check(value, expected, expected2=None, eq=self.assertEqual):
+            if expected2 is None:
+                expected2 = expected
             del result[:]
             r = self.interp.call('testfunc', value)
             self.assertEqual(len(result), 1)
-            self.assertIsInstance(result[0], str)
-            eq(result[0], expected)
-            self.assertIsInstance(r, str)
-            eq(r, expected)
+            self.assertIsInstance(result[0], (str, unicode))
+            eq(result[0], expected2)
+            self.assertIsInstance(r, (str, unicode))
+            eq(r, expected2)
         def float_eq(actual, expected):
             expected = float(expected)
             self.assertAlmostEqual(float(actual), expected,
@@ -376,7 +400,15 @@
         check(False, '0')
         check('string', 'string')
         check('string\xbd', 'string\xbd')
-        check('string\u20ac', 'string\u20ac')
+        check('string\xe2\x82\xac', 'string\xe2\x82\xac', u'string\u20ac')
+        check(u'string', u'string')
+        check(u'string\xbd', 'string\xc2\xbd', u'string\xbd')
+        check(u'string\u20ac', 'string\xe2\x82\xac', u'string\u20ac')
+        check('str\xc0\x80ing', 'str\xc0\x80ing', u'str\x00ing')
+        check('str\xc0\x80ing\xe2\x82\xac', 'str\xc0\x80ing\xe2\x82\xac', u'str\x00ing\u20ac')
+        check(u'str\x00ing', 'str\xc0\x80ing', u'str\x00ing')
+        check(u'str\x00ing\xbd', 'str\xc0\x80ing\xc2\xbd', u'str\x00ing\xbd')
+        check(u'str\x00ing\u20ac', 'str\xc0\x80ing\xe2\x82\xac', u'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):
@@ -405,6 +437,7 @@
             (u'a\n b\t\r c\n ', ('a', 'b', 'c')),
             ('a \xe2\x82\xac', ('a', '\xe2\x82\xac')),
             (u'a \u20ac', ('a', '\xe2\x82\xac')),
+            ('a\xc0\x80b c\xc0\x80d', ('a\xc0\x80b', 'c\xc0\x80d')),
             ('a {b c}', ('a', 'b c')),
             (r'a b\ c', ('a', 'b c')),
             (('a', 'b c'), ('a', 'b c')),
@@ -449,6 +482,8 @@
             (u'a\n b\t\r c\n ', ('a', 'b', 'c')),
             ('a \xe2\x82\xac', ('a', '\xe2\x82\xac')),
             (u'a \u20ac', ('a', '\xe2\x82\xac')),
+            ('a\xc0\x80b', 'a\xc0\x80b'),
+            ('a\xc0\x80b c\xc0\x80d', ('a\xc0\x80b', 'c\xc0\x80d')),
             ('a {b c}', ('a', ('b', 'c'))),
             (r'a b\ c', ('a', ('b', 'c'))),
             (('a', 'b c'), ('a', ('b', 'c'))),
diff --git a/Misc/NEWS b/Misc/NEWS
index 48a0085..2b1dd07 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -38,6 +38,9 @@
 Library
 -------
 
+- Issue #20368: The null character now correctly passed from Tcl to Python (in
+  unicode strings only).  Improved error handling in variables-related commands.
+
 - Issue #20435: Fix _pyio.StringIO.getvalue() to take into account newline
   translation settings.
 
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 0627d60..8d52b89 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -456,6 +456,68 @@
 
 
 
+#ifdef Py_USING_UNICODE
+static PyObject *
+unicode_FromTclStringAndSize(const char *s, Py_ssize_t size)
+{
+    PyObject *r = PyUnicode_DecodeUTF8(s, size, NULL);
+    if (!r && PyErr_ExceptionMatches(PyExc_UnicodeDecodeError)) {
+        /* Tcl encodes null character as \xc0\x80 */
+        if (memchr(s, '\xc0', size)) {
+            char *buf, *q;
+            const char *e = s + size;
+            PyErr_Clear();
+            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, NULL);
+            PyMem_Free(buf);
+        }
+    }
+    return r;
+}
+#endif
+
+static PyObject *
+fromTclStringAndSize(const char *s, Py_ssize_t size)
+{
+    PyObject *r;
+#ifdef Py_USING_UNICODE
+    Py_ssize_t i;
+    /* If Tcl string contains any bytes with the top bit set,
+        it's UTF-8 and we should decode it to Unicode */
+    for (i = 0; i < size; i++)
+        if (s[i] & 0x80)
+            break;
+    if (i != size) {
+        /* It isn't an ASCII string. */
+        r = unicode_FromTclStringAndSize(s, size);
+        if (r)
+            return r;
+        PyErr_Clear();
+    }
+#endif
+    r = PyString_FromStringAndSize(s, size);
+    return r;
+}
+
+static PyObject *
+fromTclString(const char *s)
+{
+    return fromTclStringAndSize(s, strlen(s));
+}
+
+
 static PyObject *
 Split(char *list)
 {
@@ -841,27 +903,10 @@
 static PyObject *
 PyTclObject_string(PyTclObject *self, void *ignored)
 {
-    char *s;
-    int i, len;
     if (!self->string) {
-        s = Tcl_GetStringFromObj(self->value, &len);
-        for (i = 0; i < len; i++)
-            if (s[i] & 0x80)
-                break;
-#ifdef Py_USING_UNICODE
-        if (i == len)
-            /* It is an ASCII string. */
-            self->string = PyString_FromStringAndSize(s, len);
-        else {
-            self->string = PyUnicode_DecodeUTF8(s, len, "strict");
-            if (!self->string) {
-                PyErr_Clear();
-                self->string = PyString_FromStringAndSize(s, len);
-            }
-        }
-#else
-        self->string = PyString_FromStringAndSize(s, len);
-#endif
+        int len;
+        char *s = Tcl_GetStringFromObj(self->value, &len);
+        self->string = fromTclStringAndSize(s, len);
         if (!self->string)
             return NULL;
     }
@@ -883,7 +928,7 @@
     }
     /* XXX Could chache result if it is non-ASCII. */
     s = Tcl_GetStringFromObj(self->value, &len);
-    return PyUnicode_DecodeUTF8(s, len, "strict");
+    return unicode_FromTclStringAndSize(s, len);
 }
 #endif
 
@@ -1022,6 +1067,8 @@
             PyErr_SetString(PyExc_OverflowError, "string is too long");
             return NULL;
         }
+        if (sizeof(Py_UNICODE) == sizeof(Tcl_UniChar))
+            return Tcl_NewUnicodeObj(inbuf, size);
         allocsize = ((size_t)size) * sizeof(Tcl_UniChar);
         if (allocsize >= size)
             outbuf = (Tcl_UniChar*)ckalloc(allocsize);
@@ -1073,30 +1120,7 @@
     TkappObject *app = (TkappObject*)tkapp;
 
     if (value->typePtr == NULL) {
-        /* If the result contains any bytes with the top bit set,
-           it's UTF-8 and we should decode it to Unicode */
-#ifdef Py_USING_UNICODE
-        int i;
-        char *s = value->bytes;
-        int len = value->length;
-        for (i = 0; i < len; i++) {
-            if (value->bytes[i] & 0x80)
-                break;
-        }
-
-        if (i == value->length)
-            result = PyString_FromStringAndSize(s, len);
-        else {
-            /* Convert UTF-8 to Unicode string */
-            result = PyUnicode_DecodeUTF8(s, len, "strict");
-            if (result == NULL) {
-                PyErr_Clear();
-                result = PyString_FromStringAndSize(s, len);
-            }
-        }
-#else
-        result = PyString_FromStringAndSize(value->bytes, value->length);
-#endif
+        result = fromTclStringAndSize(value->bytes, value->length);
         return result;
     }
 
@@ -1273,8 +1297,8 @@
 Tkapp_CallResult(TkappObject *self)
 {
     PyObject *res = NULL;
+    Tcl_Obj *value = Tcl_GetObjResult(self->interp);
     if(self->wantobjects) {
-        Tcl_Obj *value = Tcl_GetObjResult(self->interp);
         /* Not sure whether the IncrRef is necessary, but something
            may overwrite the interpreter result while we are
            converting it. */
@@ -1282,33 +1306,9 @@
         res = FromObj((PyObject*)self, value);
         Tcl_DecrRefCount(value);
     } else {
-        const char *s = Tcl_GetStringResult(self->interp);
-        const char *p = s;
-
-        /* If the result contains any bytes with the top bit set,
-           it's UTF-8 and we should decode it to Unicode */
-#ifdef Py_USING_UNICODE
-        while (*p != '\0') {
-            if (*p & 0x80)
-                break;
-            p++;
-        }
-
-        if (*p == '\0')
-            res = PyString_FromStringAndSize(s, (int)(p-s));
-        else {
-            /* Convert UTF-8 to Unicode string */
-            p = strchr(p, '\0');
-            res = PyUnicode_DecodeUTF8(s, (int)(p-s), "strict");
-            if (res == NULL) {
-                PyErr_Clear();
-                res = PyString_FromStringAndSize(s, (int)(p-s));
-            }
-        }
-#else
-        p = strchr(p, '\0');
-        res = PyString_FromStringAndSize(s, (int)(p-s));
-#endif
+        int len;
+        const char *s = Tcl_GetStringFromObj(value, &len);
+        res = fromTclStringAndSize(s, len);
     }
     return res;
 }
@@ -1611,16 +1611,28 @@
 static int
 varname_converter(PyObject *in, void *_out)
 {
+    char *s;
     char **out = (char**)_out;
     if (PyString_Check(in)) {
-        *out = PyString_AsString(in);
+        if (PyString_Size(in) > INT_MAX) {
+            PyErr_SetString(PyExc_OverflowError, "string is too long");
+            return 0;
+        }
+        s = PyString_AsString(in);
+        if (strlen(s) != PyString_Size(in)) {
+            PyErr_SetString(PyExc_ValueError, "null character in string");
+            return 0;
+        }
+        *out = s;
         return 1;
     }
     if (PyTclObject_Check(in)) {
         *out = PyTclObject_TclString(in);
         return 1;
     }
-    /* XXX: Should give diagnostics. */
+    PyErr_Format(PyExc_TypeError,
+                 "must be str or Tcl_Obj, not %.50s",
+                 in->ob_type->tp_name);
     return 0;
 }
 
@@ -1706,8 +1718,11 @@
     PyObject *res = NULL;
     Tcl_Obj *newval, *ok;
 
-    if (PyArg_ParseTuple(args, "O&O:setvar",
-                         varname_converter, &name1, &newValue)) {
+    switch (PyTuple_GET_SIZE(args)) {
+    case 2:
+        if (!PyArg_ParseTuple(args, "O&O:setvar",
+                              varname_converter, &name1, &newValue))
+            return NULL;
         /* XXX Acquire tcl lock??? */
         newval = AsObj(newValue);
         if (newval == NULL)
@@ -1723,27 +1738,27 @@
             Py_INCREF(res);
         }
         LEAVE_OVERLAP_TCL
-    }
-    else {
-        PyErr_Clear();
-        if (PyArg_ParseTuple(args, "ssO:setvar",
-                             &name1, &name2, &newValue)) {
-            /* XXX must hold tcl lock already??? */
-            newval = AsObj(newValue);
-            ENTER_TCL
-            ok = Tcl_SetVar2Ex(Tkapp_Interp(self), name1, name2, newval, flags);
-            ENTER_OVERLAP
-            if (!ok)
-                Tkinter_Error(self);
-            else {
-                res = Py_None;
-                Py_INCREF(res);
-            }
-            LEAVE_OVERLAP_TCL
-        }
-        else {
+        break;
+    case 3:
+        if (!PyArg_ParseTuple(args, "ssO:setvar",
+                              &name1, &name2, &newValue))
             return NULL;
+        /* XXX must hold tcl lock already??? */
+        newval = AsObj(newValue);
+        ENTER_TCL
+        ok = Tcl_SetVar2Ex(Tkapp_Interp(self), name1, name2, newval, flags);
+        ENTER_OVERLAP
+        if (!ok)
+            Tkinter_Error(self);
+        else {
+            res = Py_None;
+            Py_INCREF(res);
         }
+        LEAVE_OVERLAP_TCL
+        break;
+    default:
+        PyErr_SetString(PyExc_TypeError, "setvar requires 2 to 3 arguments");
+        return NULL;
     }
     return res;
 }
@@ -1783,7 +1798,9 @@
             res = FromObj(self, tres);
         }
         else {
-            res = PyString_FromString(Tcl_GetString(tres));
+            int len;
+            char *s = Tcl_GetStringFromObj(tres, &len);
+            res = PyString_FromStringAndSize(s, len);
         }
     }
     LEAVE_OVERLAP_TCL
@@ -1921,7 +1938,7 @@
     if (retval == TCL_ERROR)
         res = Tkinter_Error(self);
     else
-        res = Py_BuildValue("s", Tkapp_Result(self));
+        res = PyString_FromString(Tkapp_Result(self));
     LEAVE_OVERLAP_TCL
     return res;
 }
@@ -2158,7 +2175,7 @@
         return PythonCmd_Error(interp);
 
     for (i = 0; i < (argc - 1); i++) {
-        PyObject *s = PyString_FromString(argv[i + 1]);
+        PyObject *s = fromTclString(argv[i + 1]);
         if (!s || PyTuple_SetItem(arg, i, s)) {
             Py_DECREF(arg);
             return PythonCmd_Error(interp);