diff --git a/pitfall/restructure/decodestream.rkt b/pitfall/restructure/decodestream.rkt
index 35bc60ab..6a4c7a6d 100644
--- a/pitfall/restructure/decodestream.rkt
+++ b/pitfall/restructure/decodestream.rkt
@@ -3,17 +3,22 @@
 
 ;; approximates https://github.com/mbutterick/restructure/blob/master/src/DecodeStream.coffee
 
-(define TYPES (let-values ([(intkeys intvalues)
-                            (for*/lists (intkeys intvalues)
-                                        ([signed (in-list '(U ""))]
-                                         [size (in-list '(8 16 24 32))]
-                                         [endian (in-list '("" BE LE))])
-                                        (values
-                                         (string->symbol (format "~aInt~a~a" signed size endian))
-                                         (/ size 8)))])
-                (for/hash ([key (in-list (append '(Float Double) intkeys))]
-                           [value (in-list (append '(4 8) intvalues))])
-                          (values key value))))
+(provide (rename-out [type-sizes TYPES]))
+
+(define type-sizes (let-values ([(intkeys intvalues)
+                                 (for*/lists (intkeys intvalues)
+                                             ([signed (in-list '(U ""))]
+                                              [size (in-list '(8 16 24 32))])
+                                             (values
+                                              (format "~aInt~a" signed size)
+                                              (/ size 8)))])
+                     (for/hash ([key (in-list (append '(Float Double) intkeys))]
+                                [value (in-list (append '(4 8) intvalues))]
+                                #:when key
+                                [endian '("" BE LE)])
+                               (values (string->symbol (format "~a~a" key endian)) value))))
+
+
 
 (define-subclass object% (DecodeStream [buffer #""])
   (field [pos 0]
diff --git a/pitfall/restructure/number.rkt b/pitfall/restructure/number.rkt
index 86d16d49..5c2054df 100644
--- a/pitfall/restructure/number.rkt
+++ b/pitfall/restructure/number.rkt
@@ -15,7 +15,9 @@
  (check-false (unsigned-type? 'Int16)))
 
 (define-subclass BinaryIO% (NumberT type [endian (if (system-big-endian?) 'BE 'LE)])
-  (getter-field [fn (format "~a~a" type (if (ends-with-8? type) "" endian))])
+  (getter-field [fn (string->symbol (format "~a~a" type (if (ends-with-8? type) "" endian)))])
+  (unless (hash-has-key? type-sizes fn)
+    (raise-argument-error 'NumberT "valid type and endian" (format "~v ~v" type endian)))
 
   (define/override (decode stream)
     (define bstr (read-bytes-exact (size) stream))
@@ -25,7 +27,7 @@
 
   (define/override (encode op val) 'foo)
 
-  (define/override (size) (hash-ref TYPES type)))
+  (define/override (size) (hash-ref type-sizes fn)))
     
 
 (test-module