diff --git a/collects/compiler/zo-marshal.rkt b/collects/compiler/zo-marshal.rkt
index 76a5731737..e9403b252c 100644
--- a/collects/compiler/zo-marshal.rkt
+++ b/collects/compiler/zo-marshal.rkt
@@ -356,7 +356,9 @@
     [(struct certificate:nest (m1 m2))
      (list* (encode-mark-map m1) (encode-mark-map m2))]
     [(struct certificate:ref (val m))
-     (list* #f (make-protected-symref val) (encode-mark-map m))]))
+     (list* #f (make-protected-symref val) (encode-mark-map m))]
+    [(struct certificate:plain (m))
+     (encode-mark-map m)]))
 
 (define (encode-wrapped w)
   (match w
diff --git a/collects/compiler/zo-parse.rkt b/collects/compiler/zo-parse.rkt
index 6a8edd6032..e6e677700e 100644
--- a/collects/compiler/zo-parse.rkt
+++ b/collects/compiler/zo-parse.rkt
@@ -524,7 +524,9 @@
         (symtab-lookup cp symref)
         (decode-mark-map alist))]
       [(list* (? list? nested) alist)
-       (make-certificate:nest (decode-mark-map nested) (decode-mark-map alist))])))
+       (make-certificate:nest (decode-mark-map nested) (decode-mark-map alist))]
+      [alist
+       (make-certificate:plain (decode-mark-map alist))])))
 
 (define stx-memo (make-memo))
 ; XXX More memo use
diff --git a/collects/compiler/zo-structs.rkt b/collects/compiler/zo-structs.rkt
index acb2476831..4aba9f8ded 100644
--- a/collects/compiler/zo-structs.rkt
+++ b/collects/compiler/zo-structs.rkt
@@ -63,6 +63,9 @@
 (define-form-struct (certificate:ref certificate)
   ([val any/c]
    [map  mark-map?]))
+(define-form-struct (certificate:plain certificate)
+  ([map  mark-map?]))
+
 
 (define-form-struct wrap ())
 (define-form-struct wrapped ([datum any/c]