diff --git a/configure.ac b/configure.ac
index 63891d0..42be29b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,7 +26,8 @@ AC_CONFIG_SRCDIR(src/integer/gcd/cl_I_gcd.cc)
 AC_CONFIG_AUX_DIR([autoconf])
 AC_CONFIG_HEADERS(autoconf/cl_config.h include/cln/config.h include/cln/host_cpu.h include/cln/version.h src/base/cl_base_config.h src/base/cl_gmpconfig.h src/timing/cl_t_config.h)
 AM_INIT_AUTOMAKE([foreign 1.8 dist-bzip2])
-AH_TOP([#ifdef _CL_CONFIG_H])
+AH_TOP([#ifndef _CL_CONFIG_H])
+AH_TOP([#define _CL_CONFIG_H])
 AH_TOP([#include "cln/host_cpu.h"])
 AH_TOP([/* prevents cln/config.h from being included, so no macro gets redefined */])
 AH_TOP([#define _CL_CONFIG_PUBLIC_H])