diff --git a/scsh/afs/interfaces.scm b/scsh/afs/interfaces.scm
index a9835d5..52ae7a7 100644
--- a/scsh/afs/interfaces.scm
+++ b/scsh/afs/interfaces.scm
@@ -1,6 +1,7 @@
 (define-interface afs-fs-interface
   (export
    ((afs-permission afs-permissions) :syntax)
+   afs-permissions?
    all-afs-permissions
    make-afs-permissions
    afs-permissions<=?