diff --git a/config/kernel.m4 b/config/kernel.m4
index 6f5d31d2d7..9c65b0feec 100644
--- a/config/kernel.m4
+++ b/config/kernel.m4
@@ -285,7 +285,7 @@ dnl #
 dnl # ZFS_LINUX_CONFTEST
 dnl #
 AC_DEFUN([ZFS_LINUX_CONFTEST], [
-cat >conftest.c <<_ACEOF
+cat confdefs.h - <<_ACEOF >conftest.c
 $1
 _ACEOF
 ])