diff -r 87dfadd61e0d Modules/_ctypes/callproc.c
--- a/Modules/_ctypes/callproc.c	Thu Feb 04 01:24:56 2016 -0500
+++ b/Modules/_ctypes/callproc.c	Fri Feb 05 00:39:56 2016 +0100
@@ -1606,6 +1606,7 @@
     if ((size_t)size <= sizeof(obj->b_value)) {
         /* internal default buffer is large enough */
         obj->b_size = size;
+        obj->b_length = size;
         goto done;
     }
     if (!_CDataObject_HasExternalBuffer(obj)) {
@@ -1618,12 +1619,14 @@
         memmove(ptr, obj->b_ptr, obj->b_size);
         obj->b_ptr = ptr;
         obj->b_size = size;
+        obj->b_length = size;
     } else {
         void * ptr = PyMem_Realloc(obj->b_ptr, size);
         if (ptr == NULL)
             return PyErr_NoMemory();
         obj->b_ptr = ptr;
         obj->b_size = size;
+        obj->b_length = size;
     }
   done:
     Py_INCREF(Py_None);