diff --git a/configure.ac b/configure.ac
index 0ed4d2d7..3eeb446d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -120,7 +120,7 @@ AC_SEARCH_LIBS([clock_gettime], [rt posix4],
 AC_CANONICAL_HOST
 
 # Store this value because ac_test_CFLAGS is overwritten by AC_PROG_CC
-if test "$ac_test_CFLAGS" != set ; then
+if ! test "$ac_test_CFLAGS" ; then
   bird_cflags_default=yes
 fi