diff --git a/gnu/build/file-systems.scm b/gnu/build/file-systems.scm
index 66ca22d6ea..36a59f5f5c 100644
--- a/gnu/build/file-systems.scm
+++ b/gnu/build/file-systems.scm
@@ -1123,6 +1123,7 @@ an exception in such cases but perform the nearest sane action."
   "Return the number suitable for the 'flags' argument of 'mount' that
 corresponds to the symbols listed in FLAGS."
   (let loop ((flags flags))
+    ;; Note: Keep in sync with ‘invalid-file-system-flags’.
     (match flags
       (('read-only rest ...)
        (logior MS_RDONLY (loop rest)))
diff --git a/gnu/system/file-systems.scm b/gnu/system/file-systems.scm
index f2eb2e0837..85b00a4a09 100644
--- a/gnu/system/file-systems.scm
+++ b/gnu/system/file-systems.scm
@@ -122,7 +122,7 @@
     ;; Note: Keep in sync with 'mount-flags->bit-mask'.
     (let ((known-flags '(read-only
                          bind-mount no-suid no-dev no-exec
-                         no-atime strict-atime lazy-time
+                         no-atime no-diratime strict-atime lazy-time
                          shared)))
       (lambda (flags)
         "Return the subset of FLAGS that is invalid."