diff --git a/syndicate/smart-pattern.rkt b/syndicate/smart-pattern.rkt
index 4855b73..379b82c 100644
--- a/syndicate/smart-pattern.rkt
+++ b/syndicate/smart-pattern.rkt
@@ -32,10 +32,13 @@
       (hash-set entries (- npats 1) (Pattern-discard))
       entries))
 
-(define (entries->items entries on-missing)
-  (define max-key (apply max -1 (hash-keys entries)))
-  (for/list [(i (in-range 0 (+ max-key 1)))]
-    (hash-ref entries i on-missing)))
+(define (entries->items entries)
+  (let/ec return
+    (define keys (hash-keys entries))
+    (for [(k (in-list keys))] (unless (exact-nonnegative-integer? k) (return (void))))
+    (define max-key (apply max -1 keys))
+    (for/list [(i (in-range 0 (+ max-key 1)))]
+      (hash-ref entries i (lambda () (return void))))))
 
 (define (rec label pats) (Pattern-group (GroupType-rec label) (items->entries pats)))
 (define (arr pats) (Pattern-group (GroupType-arr) (items->entries pats)))
@@ -57,7 +60,10 @@
 
 (define (literal-pattern->literal p)
   (let/ec return
-    (define (e->i e) (entries->items e (lambda () (return (void)))))
+    (define (e->i e)
+      (match (entries->items e)
+        [(? void?) (return (void))]
+        [items items]))
     (let walk ((p p))
       (match p
         [(Pattern-discard) (return (void))]