correct phrasing of conditional for security sources

diff --git a/Makefile b/Makefile
index e11557ba4d019671f3da77ef9f3107556c6855a5..1d8d694ca7038f5ba7d98d8fc94cc78334744f69 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -41,7 +41,7 @@ KEYRING := keys/debian-archive-${release}-automatic.gpg
 # KEYRING := keys/debian-archive-${release}-security-automatic.gpg
 
 ifndef SOURCES
-  ifdef $(filter stretch jessie buster, ${release})
+  ifneq "$(filter stretch jessie buster, ${release})" ""
     define SOURCES := 
     deb        http://cdn.debian.net/debian    ${release}      main
     deb        https://deb.debian.org/debian-security/ ${release}/updates      main