diff --git a/collects/ffi/unsafe.rkt b/collects/ffi/unsafe.rkt
index f320bf8e08..141f766e8c 100644
--- a/collects/ffi/unsafe.rkt
+++ b/collects/ffi/unsafe.rkt
@@ -1143,7 +1143,7 @@
 
 ;; Simple structs: call this with a list of types, and get a type that marshals
 ;; C structs to/from Scheme lists.
-(define* (_list-struct #:alignment [alignment 2] . types)
+(define* (_list-struct #:alignment [alignment #f] . types)
   (let ([stype   (make-cstruct-type types #f alignment)]
         [offsets (compute-offsets types)]
         [len     (length types)])