diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index fd28aa9..bdc9f00 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -144,7 +144,8 @@
 
 static PyObject *
 IO_isatty(IOobject *self, PyObject *unused) {
-	Py_INCREF(Py_False);
+        if (!IO__opencheck(self)) return NULL;
+        Py_INCREF(Py_False);
         return Py_False;
 }