diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
index 79fc6c8..98c609c 100644
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -490,7 +490,7 @@
 			      &obj, &offset))
 		return NULL;
 
-	if (-1 == PyObject_AsReadBuffer(obj, &buffer, &buffer_len))
+	if (-1 == PyObject_AsReadBuffer(obj, (const void**)&buffer, &buffer_len))
 		return NULL;
 
 	if (offset < 0) {