diff --git a/src/config.in b/src/config.in
index 204c7d85dfe..34bd389289a 100644
--- a/src/config.in
+++ b/src/config.in
@@ -305,6 +305,7 @@ Boston, MA 02111-1307, USA.  */
 #undef HAVE_AIX_SMT_EXP
 
 #undef vfork
+#undef size_t
 
 /* Define if you have the ANSI `strerror' function.
    Otherwise you must have the variable `char *sys_errlist[]'.  */