diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index 03ef461..100891b 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -657,7 +657,7 @@
   char *buf;
   Py_ssize_t size;
 
-  if (PyObject_AsCharBuffer(s, (const void **)&buf, &size) != 0)
+  if (PyObject_AsCharBuffer(s, (const char **)&buf, &size) != 0)
       return NULL;
 
   self = PyObject_New(Iobject, &Itype);