diff --git a/m4/dune_unused.m4 b/m4/dune_unused.m4
index db72bf260908ee28070404b31eb35464f24d07d6..1147bc3b25654e1afc043e2aa694f38ce696e7f6 100644
--- a/m4/dune_unused.m4
+++ b/m4/dune_unused.m4
@@ -3,11 +3,12 @@
 AC_DEFUN([DUNE_CHECKUNUSED],[
     AC_CACHE_CHECK([for __attribute__((unused))], dune_cv_attribute_unused, [
         AC_LANG_PUSH([C++])
-        AC_TRY_COMPILE([#define UNUSED __attribute__((unused))],
-                       [void f(int a UNUSED, int UNUSED)
+        AC_TRY_COMPILE([#define UNUSED __attribute__((unused))
+                        void f(int a UNUSED, int UNUSED)
                         {
                           int UNUSED b;
                         }],
+                       [],
                         dune_cv_attribute_unused="yes",
                         dune_cv_attribute_unused="no")
         AC_LANG_POP([C++])