diff --git a/release/src/router/tinc/configure.ac b/release/src/router/tinc/configure.ac
index 4c0f9260c1..4a80d25010 100644
--- a/release/src/router/tinc/configure.ac
+++ b/release/src/router/tinc/configure.ac
@@ -142,7 +142,7 @@ dnl No -fstack-protector-all because it doesn't work on all platforms or archite
 
 AC_ARG_ENABLE([hardening], AS_HELP_STRING([--disable-hardening], [disable compiler and linker hardening flags]))
 AS_IF([test "x$enable_hardening" != "xno"],
-  [AX_CHECK_COMPILE_FLAG([-DFORTIFY_SOURCE=2], [CPPFLAGS="$CPPFLAGS -DFORTIFY_SOURCE=2"])
+  [AX_CHECK_COMPILE_FLAG([-D_FORTIFY_SOURCE=2], [CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"])
    AX_CHECK_COMPILE_FLAG([-fwrapv], [CPPFLAGS="$CPPFLAGS -fwrapv"],
    AX_CHECK_COMPILE_FLAG([-fno-strict-overflow], [CPPFLAGS="$CPPFLAGS -fno-strict-overflow"]))
    case $host_os in