diff --git a/_imagingtk.c b/_imagingtk.c
index 87de36a04..d0295f317 100644
--- a/_imagingtk.c
+++ b/_imagingtk.c
@@ -36,18 +36,18 @@ _tkinit(PyObject* self, PyObject* args)
 {
     Tcl_Interp* interp;
 
-    Py_ssize_t arg;
+    PyObject* arg;
     int is_interp;
-    if (!PyArg_ParseTuple(args, "ni", &arg, &is_interp))
+    if (!PyArg_ParseTuple(args, "Oi", &arg, &is_interp))
         return NULL;
 
     if (is_interp)
-        interp = (Tcl_Interp*) arg;
+        interp = (Tcl_Interp*)PyLong_AsVoidPtr(arg);
     else {
         TkappObject* app;
 	/* Do it the hard way.  This will break if the TkappObject
 	   layout changes */
-        app = (TkappObject*) arg;
+        app = (TkappObject*)PyLong_AsVoidPtr(arg);
         interp = app->interp;
     }