diff --git a/pisi/specfile.py b/pisi/specfile.py
index fc699b64..15283d51 100644
--- a/pisi/specfile.py
+++ b/pisi/specfile.py
@@ -151,7 +151,7 @@ class Source:
     t_Packager = [Packager, autoxml.mandatory]
     t_Summary = [autoxml.LocalText, autoxml.mandatory]
     t_Description = [autoxml.LocalText, autoxml.mandatory]
-    t_IsA = [ [autoxml.String], autoxml.mandatory]
+    t_IsA = [ [autoxml.String], autoxml.optional]
     t_PartOf = [autoxml.String, autoxml.optional]
     t_Icon = [ autoxml.String, autoxml.optional]
     t_License = [ [autoxml.String], autoxml.mandatory]