diff --git a/pisi-spec.rng b/pisi-spec.rng
index d937a214..a31eb14d 100644
--- a/pisi-spec.rng
+++ b/pisi-spec.rng
@@ -295,6 +295,7 @@
                 <value>GPLv2-with-linking-exception</value>
                 <value>GPLv3</value>
                 <value>GPLv3+</value>
+                <value>GFDLv1</value>
                 <value>HBRL</value>
                 <value>HKSCS</value>
                 <value>HPL</value>