diff -r 92b292d68104 Lib/test/test_tcl.py
--- a/Lib/test/test_tcl.py	Mon Sep 08 17:29:02 2014 -0400
+++ b/Lib/test/test_tcl.py	Tue Sep 09 17:58:43 2014 +0300
@@ -418,7 +418,6 @@
         self.assertEqual(passValue(['a', ['b', 'c']]),
                          ('a', ('b', 'c')) if self.wantobjects else 'a {b c}')
 
-    @unittest.skipIf(sys.platform.startswith("aix"), 'Issue #21951: crashes on AIX')
     def test_user_command(self):
         result = None
         def testfunc(arg):
@@ -446,9 +445,11 @@
         check('string')
         check('string\xbd')
         check('string\u20ac')
+        check('')
         check(b'string', 'string')
         check(b'string\xe2\x82\xac', 'string\xe2\x82\xac')
         check(b'string\xbd', 'string\xbd')
+        check(b'', '')
         check('str\x00ing')
         check('str\x00ing\xbd')
         check('str\x00ing\u20ac')
diff -r 92b292d68104 Modules/_tkinter.c
--- a/Modules/_tkinter.c	Mon Sep 08 17:29:02 2014 -0400
+++ b/Modules/_tkinter.c	Tue Sep 09 17:58:43 2014 +0300
@@ -605,7 +605,7 @@
         Tcl_SetVar(v->interp, "tcl_interactive", "0", TCL_GLOBAL_ONLY);
 
     /* This is used to get the application class for Tk 4.1 and up */
-    argv0 = (char*)ckalloc(strlen(className) + 1);
+    argv0 = (char*)attemptckalloc(strlen(className) + 1);
     if (!argv0) {
         PyErr_NoMemory();
         Py_DECREF(v);
@@ -639,7 +639,7 @@
         if (use)
             len += strlen(use) + sizeof "-use ";
 
-        args = (char*)ckalloc(len);
+        args = (char*)attemptckalloc(len);
         if (!args) {
             PyErr_NoMemory();
             Py_DECREF(v);
@@ -906,13 +906,15 @@
         Py_ssize_t size, i;
 
         size = PySequence_Fast_GET_SIZE(value);
+        if (size == 0)
+            return Tcl_NewListObj(0, NULL);
         if (!CHECK_SIZE(size, sizeof(Tcl_Obj *))) {
             PyErr_SetString(PyExc_OverflowError,
                             PyTuple_Check(value) ? "tuple is too long" :
                                                    "list is too long");
             return NULL;
         }
-        argv = (Tcl_Obj **) ckalloc(((size_t)size) * sizeof(Tcl_Obj *));
+        argv = (Tcl_Obj **) attemptckalloc(((size_t)size) * sizeof(Tcl_Obj *));
         if(!argv) {
           PyErr_NoMemory();
           return NULL;
@@ -936,6 +938,8 @@
 
         inbuf = PyUnicode_DATA(value);
         size = PyUnicode_GET_LENGTH(value);
+        if (size == 0)
+            return Tcl_NewUnicodeObj((const void *)"", 0);
         if (!CHECK_SIZE(size, sizeof(Tcl_UniChar))) {
             PyErr_SetString(PyExc_OverflowError, "string is too long");
             return NULL;
@@ -944,7 +948,7 @@
         if (kind == sizeof(Tcl_UniChar))
             return Tcl_NewUnicodeObj(inbuf, size);
         allocsize = ((size_t)size) * sizeof(Tcl_UniChar);
-        outbuf = (Tcl_UniChar*)ckalloc(allocsize);
+        outbuf = (Tcl_UniChar*)attemptckalloc(allocsize);
         /* Else overflow occurred, and we take the next exit */
         if (!outbuf) {
             PyErr_NoMemory();
@@ -1111,7 +1115,7 @@
                                                       "list is too long");
                 return NULL;
             }
-            objv = (Tcl_Obj **)ckalloc(((size_t)objc) * sizeof(Tcl_Obj *));
+            objv = (Tcl_Obj **)attemptckalloc(((size_t)objc) * sizeof(Tcl_Obj *));
             if (objv == NULL) {
                 PyErr_NoMemory();
                 objc = 0;
@@ -1247,7 +1251,7 @@
         PyObject *exc_type, *exc_value, *exc_tb;
         if (!WaitForMainloop(self))
             return NULL;
-        ev = (Tkapp_CallEvent*)ckalloc(sizeof(Tkapp_CallEvent));
+        ev = (Tkapp_CallEvent*)attemptckalloc(sizeof(Tkapp_CallEvent));
         ev->ev.proc = (Tcl_EventProc*)Tkapp_CallProc;
         ev->self = self;
         ev->args = args;
@@ -1498,7 +1502,7 @@
         if (!WaitForMainloop(self))
             return NULL;
 
-        ev = (VarEvent*)ckalloc(sizeof(VarEvent));
+        ev = (VarEvent*)attemptckalloc(sizeof(VarEvent));
 
         ev->self = selfptr;
         ev->args = args;
@@ -2098,7 +2102,7 @@
 #ifdef WITH_THREAD
     if (self->threaded && self->thread_id != Tcl_GetCurrentThread()) {
         Tcl_Condition cond = NULL;
-        CommandEvent *ev = (CommandEvent*)ckalloc(sizeof(CommandEvent));
+        CommandEvent *ev = (CommandEvent*)attemptckalloc(sizeof(CommandEvent));
         ev->ev.proc = (Tcl_EventProc*)Tkapp_CommandProc;
         ev->interp = self->interp;
         ev->create = 1;
@@ -2144,7 +2148,7 @@
     if (self->threaded && self->thread_id != Tcl_GetCurrentThread()) {
         Tcl_Condition cond = NULL;
         CommandEvent *ev;
-        ev = (CommandEvent*)ckalloc(sizeof(CommandEvent));
+        ev = (CommandEvent*)attemptckalloc(sizeof(CommandEvent));
         ev->ev.proc = (Tcl_EventProc*)Tkapp_CommandProc;
         ev->interp = self->interp;
         ev->create = 0;