diff --git a/guix-data-service/model/nar.scm b/guix-data-service/model/nar.scm
index d275421..c66e6d2 100644
--- a/guix-data-service/model/nar.scm
+++ b/guix-data-service/model/nar.scm
@@ -375,7 +375,7 @@ ORDER BY COUNT(*) DESC")
                  (match status
                    ("t" 'matching)
                    ("f" 'not-matching)
-                   (() 'unknown))
+                   ((? NULL? x) 'unknown))
                  (string->number count))))
         (exec-query-with-null-handling conn query (list revision-commit)))))