diff -r 2c8062da7250 Lib/ctypes/__init__.py
--- a/Lib/ctypes/__init__.py	Sat Jan 09 12:22:13 2016 -0500
+++ b/Lib/ctypes/__init__.py	Sat Jan 09 23:01:30 2016 +0200
@@ -52,6 +52,8 @@
     if isinstance(init, bytes):
         if size is None:
             size = len(init)+1
+        elif len(init) >= size and init[size - 1] != 0:
+            raise ValueError("Buffer size is too small to contain initial value and NULL char.")
         buftype = c_char * size
         buf = buftype()
         buf.value = init