diff --git a/collects/mred/private/kernel.ss b/collects/mred/private/kernel.ss
index 794007f5b7..7c257f25ab 100644
--- a/collects/mred/private/kernel.ss
+++ b/collects/mred/private/kernel.ss
@@ -78,9 +78,9 @@
 		   (syntax
 		    (define name (let ([c (dynamic-require ''#%mred-kernel 'name)])
 				   (make-primitive-class
-				    (lambda (class prop:object preparer dispatcher more-props)
+				    (lambda (class prop:object preparer dispatcher unwrapper more-props)
 				      (kernel:primitive-class-prepare-struct-type! 
-				       c prop:object class preparer dispatcher more-props))
+				       c prop:object class preparer dispatcher unwrapper more-props))
 				    kernel:initialize-primitive-object
 				    'print-name super (list intf ...) 'args
 				    '(old ...)
diff --git a/src/mzscheme/utils/xcglue.c b/src/mzscheme/utils/xcglue.c
index 0aa7e6c8c2..54f59c101d 100644
--- a/src/mzscheme/utils/xcglue.c
+++ b/src/mzscheme/utils/xcglue.c
@@ -562,7 +562,7 @@ void objscheme_init(Scheme_Env *env)
   scheme_install_xc_global("primitive-class-prepare-struct-type!",
 			   scheme_make_prim_w_arity(class_prepare_struct_type,
 						    "primitive-class-prepare-struct-type!",
-						    6, 6),
+						    7, 7),
 			   env);
   
   scheme_install_xc_global("primitive-class-find-method",