diff --git a/tapl/stlc+occurrence.rkt b/tapl/stlc+occurrence.rkt
index 640e364..98ea25d 100644
--- a/tapl/stlc+occurrence.rkt
+++ b/tapl/stlc+occurrence.rkt
@@ -67,6 +67,7 @@
 ;; flatten nested unions
 
 (begin-for-syntax
+
   (define τ-eval (current-type-eval))
 
   (define (τ->symbol τ)
@@ -84,36 +85,51 @@
      [_
       (error 'τ->symbol (~a (syntax->datum τ)))]))
 
-  (define (∪-eval τ-stx)
-    (syntax-parse (τ-eval τ-stx)
-     [(~∪ τ-stx* ...)
-      ;; Recursively evaluate members
-      (define τ**
-        (for/list ([τ (in-list (syntax->list #'(τ-stx* ...)))])
-          (let ([τ+ (∪-eval τ)])
-            (if (∪? τ+)
-                (∪->list τ+)
-                (list τ+)))))
-      ;; Remove duplicates from the union, sort members
-      (define τ*
-        (sort
-         (remove-duplicates (apply append τ**) (current-type=?))
-         symbol<?
-         #:key τ->symbol))
-      ;; Check for empty & singleton lists
-      (define τ
-        (cond
-         [(null? τ*)
-          (raise-user-error 'τ-eval "~a (~a:~a) empty union type ~a\n"
-                            (syntax-source τ-stx) (syntax-line τ-stx) (syntax-column τ-stx)
-                            (syntax->datum τ-stx))]
-         [(null? (cdr τ*))
-          #`#,(car τ*)]
-         [else
-          #`#,(cons #'∪ τ*)]))
-      (τ-eval τ)]
-     [_
-      (τ-eval τ-stx)]))
+  (define ∪-eval 
+    ;; Private helper: check that all functions have unique arities
+    ;; It's private because it assumes all τ* have been evaluated
+    (let ([assert-unique-arity-arrows
+           (lambda (τ*)
+             (for/fold ([seen '()])
+                       ([τ (in-list τ*)])
+               (syntax-parse τ
+                [(~→ τ-dom* ... τ-cod)
+                 (define arity (stx-length #'(τ-dom* ...)))
+                 (when (memv arity seen)
+                   (error '∪ (format "Cannot discriminate types in the union ~a. Multiple functions have arity ~a." (cons '∪ (map syntax->datum τ*)) arity)))
+                 (cons arity seen)]
+                [_ seen])))])
+    (lambda (τ-stx)
+     (syntax-parse (τ-eval τ-stx)
+      [(~∪ τ-stx* ...)
+       ;; Recursively evaluate members
+       (define τ**
+         (for/list ([τ (in-list (syntax->list #'(τ-stx* ...)))])
+           (let ([τ+ (∪-eval τ)])
+             (if (∪? τ+)
+                 (∪->list τ+)
+                 (list τ+)))))
+       ;; Remove duplicates from the union, sort members
+       (define τ*
+         (sort
+          (remove-duplicates (apply append τ**) (current-type=?))
+          symbol<?
+          #:key τ->symbol))
+       ;; Check for empty & singleton lists
+       (define τ
+         (cond
+          [(null? τ*)
+           (raise-user-error 'τ-eval "~a (~a:~a) empty union type ~a\n"
+                             (syntax-source τ-stx) (syntax-line τ-stx) (syntax-column τ-stx)
+                             (syntax->datum τ-stx))]
+          [(null? (cdr τ*))
+           #`#,(car τ*)]
+          [else
+           (assert-unique-arity-arrows τ*)
+           #`#,(cons #'∪ τ*)]))
+       (τ-eval τ)]
+      [_
+       (τ-eval τ-stx)]))))
   (current-type-eval ∪-eval))
 
 ;; -----------------------------------------------------------------------------
diff --git a/tapl/tests/stlc+occurrence-tests.rkt b/tapl/tests/stlc+occurrence-tests.rkt
index e573803..f815af4 100644
--- a/tapl/tests/stlc+occurrence-tests.rkt
+++ b/tapl/tests/stlc+occurrence-tests.rkt
@@ -272,8 +272,8 @@
 ;; -----------------------------------------------------------------------------
 ;; --- Functions in union
 
-(check-type (λ ([x : (∪ Int (∪ Nat) (∪ (→ Int Int)) (→ (→ (→ Int Int)) Int))]) #t)
-            : (→ (∪ Int Nat (→ Int Int) (→ (→ (→ Int Int)) Int)) Boolean))
+(check-type (λ ([x : (∪ Int (∪ Nat) (∪ (→ Int Str Int)) (→ (→ (→ Int Int)) Int))]) #t)
+            : (→ (∪ Int Nat (→ Int Str Int) (→ (→ (→ Int Int)) Int)) Boolean))
 
 (check-type (λ ([x : (∪ Int (→ Int Int))]) #t)
             : (→ Int Boolean))
@@ -324,8 +324,8 @@
 
 ;; --- disallow same-arity functions
 (typecheck-fail
- (λ ([x : (∪ (→ Int Int) (→ Str Str))]) (x 1))
- #:with-msg "boooo")
+ (λ ([x : (∪ (→ Int Int) (→ Str Str))]) 1)
+ #:with-msg "Cannot discriminate")
 
 ;; -----------------------------------------------------------------------------
 ;; --- TODO Filter values (should do nothing)