diff --git a/pitfall/restructure/struct.rkt b/pitfall/restructure/struct.rkt
index 44343ea3..20a2fa31 100644
--- a/pitfall/restructure/struct.rkt
+++ b/pitfall/restructure/struct.rkt
@@ -91,7 +91,8 @@ https://github.com/mbutterick/restructure/blob/master/src/Struct.coffee
     (define ctx (mhasheq 'parent parent
                          'val val
                          'pointerSize 0))
-    (+ (for/sum ([(key type) (in-dict fields)])
+    (+ (for/sum ([(key type) (in-dict fields)]
+                 #:when val)
          (send type size (ref val key) ctx))
        (if include-pointers (· ctx pointerSize) 0)))