better error message

xenomorph/xenomorph/dict.rkt

@@ -76,9 +76,13 @@ https://github.com/mbutterick/restructure/blob/master/src/Struct.coffee
       (define parent (mhasheq x:parent-key parent-arg
                               x:val-key val
                               x:pointer-size-key 0))
-      (define fields-size (for/sum ([(key type) (in-dict @fields)]
+      (define fields-size (let ([key-missing-signal (gensym)])
+                            (for/sum ([(key type) (in-dict @fields)]
                                       #:when (xenomorphic-type? type))
-                            (send type x:size (and val (send type pre-encode (dict-ref val key))) parent)))
+                                     (match (dict-ref val key key-missing-signal)
+                                       [(== key-missing-signal eq?)
+                                        (raise-argument-error 'x:dict (format "dict that contains value for key ~v" key) val)]
+                                       [val (send type x:size (and val (send type pre-encode val)) parent)]))))
       (define pointers-size (if include-pointers (dict-ref parent x:pointer-size-key) 0))
       (+ fields-size pointers-size))))