diff --git a/unify.rkt b/unify.rkt
index fd6124b..77f3fef 100644
--- a/unify.rkt
+++ b/unify.rkt
@@ -2,6 +2,7 @@
 
 (require racket/set)
 (require racket/match)
+(require (only-in racket/class object?))
 (require "struct-map.rkt")
 
 (provide (struct-out variable)
@@ -57,6 +58,13 @@
 (define (non-wild? x)
   (not (wild? x)))
 
+;; Any -> Boolean
+;; Racket objects are structures, so we reject them explicitly for
+;; now, leaving them opaque to unification.
+(define (non-object-struct? x)
+  (and (struct? x)
+       (not (object? x))))
+
 ;; Any -> Set<Variable>
 (define (variables-in x)
   (let walk ((x x) (acc (set)))
@@ -64,7 +72,7 @@
      [(variable? x) (set-add acc x)]
      [(pair? x) (walk (car x) (walk (cdr x) acc))]
      [(vector? x) (foldl walk acc (vector->list x))]
-     [(struct? x) (walk (struct->vector x #f) acc)]
+     [(non-object-struct? x) (walk (struct->vector x #f) acc)]
      [else acc])))
 
 ;; Variable Any -> Boolean
@@ -74,7 +82,7 @@
      [(eq? var x) #t]
      [(pair? x) (or (walk (car x)) (walk (cdr x)))]
      [(vector? x) (ormap walk (vector->list x))]
-     [(struct? x) (walk (struct->vector x #f))]
+     [(non-object-struct? x) (walk (struct->vector x #f))]
      [else #f])))
 
 ;; Variable Any Subst -> Subst
@@ -115,7 +123,7 @@
 	     (walk (car a) (car b) (walk (cdr a) (cdr b) env))]
 	    [(and (vector? a) (vector? b) (= (vector-length a) (vector-length b)))
 	     (for/fold ([env env]) ([ea a] [eb b]) (walk ea eb env))]
-	    [(and (struct? a) (struct? b))
+	    [(and (non-object-struct? a) (non-object-struct? b))
 	     (walk (struct->vector a #f) (struct->vector b #f) env)]
 	    [else (and (equal? a b) env)])))))
 
@@ -138,7 +146,7 @@
 		       (define-values (val env1) (walk (vector-ref x i) env))
 		       (vector-set! result i val)
 		       env1))]
-     [(struct? x) (struct-map/accumulator walk env x)]
+     [(non-object-struct? x) (struct-map/accumulator walk env x)]
      [else (values x env)])))
 
 ;; Any -> Any
@@ -151,7 +159,7 @@
 	    [else (define v (canonical-variable (hash-count env))) (hash-set! env t v) v])]
      [(pair? t) (cons (walk (car t)) (walk (cdr t)))]
      [(vector? t) (list->vector (map walk (vector->list t)))]
-     [(struct? t) (struct-map walk t)]
+     [(non-object-struct? t) (struct-map walk t)]
      [else t])))
 
 ;; Any -> Boolean
@@ -197,7 +205,7 @@
 		  v])]
      [(pair? t) (cons (walk (car t)) (walk (cdr t)))]
      [(vector? t) (list->vector (map walk (vector->list t)))]
-     [(struct? t) (struct-map walk t)]
+     [(non-object-struct? t) (struct-map walk t)]
      [else t])))
 
 ;; Any -> Any
@@ -251,7 +259,7 @@
     (cond
      [(pair? x) (cons (walk (car x)) (walk (cdr x)))]
      [(vector? x) (list->vector (map walk (vector->list x)))]
-     [(struct? x) (struct-map walk x)]
+     [(non-object-struct? x) (struct-map walk x)]
      [else x])))
 
 ;; True iff a is a specialization (or instance) of b.
@@ -264,7 +272,7 @@
       (and (walk (car a) (car b)) (walk (cdr a) (cdr b)))]
      [(and (vector? a) (vector? b) (= (vector-length a) (vector-length b)))
       (for/and ([aa a] [bb b]) (walk aa bb))]
-     [(and (struct? a) (struct? b))
+     [(and (non-object-struct? a) (non-object-struct? b))
       (walk (struct->vector a #f) (struct->vector b #f))]
      [else (equal? a b)])))