diff --git a/private/optcontract.rkt b/private/optcontract.rkt
index 100a501..dca4e3a 100644
--- a/private/optcontract.rkt
+++ b/private/optcontract.rkt
@@ -1,15 +1,21 @@
 #lang racket
 
-(require (rename-in racket/contract
-                    [define/contract define/contract/always]))
-
 (provide (except-out (all-from-out racket/contract)
                      define-struct/contract
                      ;define/contract
                      provide/contract
                      invariant-assertion)
          define/contract
-         define/contract/always)
+         define/contract/always
+         define/contract/alt)
+
+(require (rename-in racket/contract
+                    [define/contract define/contract/always]))
 
 (define-syntax-rule (define/contract sig c . rest)
-  (define sig . rest))
\ No newline at end of file
+  (define sig . rest))
+
+;; The alt-code is executed in the body of the function when the contract is
+;; disabled:
+(define-syntax-rule (define/contract/alt sig c alt-code . rest)
+  (define sig alt-code . rest))
\ No newline at end of file
diff --git a/private/template-subscripts.rkt b/private/template-subscripts.rkt
index 3eabd2b..7c773b9 100644
--- a/private/template-subscripts.rkt
+++ b/private/template-subscripts.rkt
@@ -241,8 +241,23 @@
 
 (define formattable/c (or/c number? string? symbol? bytes?))
 
-(define/contract/always
-  (generate-nested-ids depth bound binder₀ format l* attribute-names whole-form)
+(define (generate-nested-ids-check-ellipsis-match-count
+         l* depth attribute-names whole-form bound)
+  (if ((ellipsis-count/c depth) l*)
+      #t
+      (raise-syntax-error
+       (syntax-case whole-form ()
+         [(self . _) (syntax-e #'self)]
+         [_ 'subtemplate])
+       "incompatible ellipsis match counts for subscripted variables:"
+       whole-form
+       bound
+       attribute-names)))
+
+(module+ test-private
+  (provide generate-nested-ids))
+
+(define generate-nested-ids-full-contract
   (->i {[depth exact-nonnegative-integer?]
         [bound identifier?]
         [binder₀ identifier?]
@@ -252,21 +267,17 @@
                                      (λ (a) (= (length l*) (length a))))]
         [whole-form syntax?]}
        #:pre (l* depth attribute-names whole-form bound)
-       (if ((ellipsis-count/c depth) l*)
-           #t
-           (raise-syntax-error
-            (syntax-case whole-form ()
-              [(self . _) (syntax-e #'self)]
-              [_ 'subtemplate])
-            "incompatible ellipsis match counts for subscripted variables:"
-            whole-form
-            bound
-            attribute-names))
+       (generate-nested-ids-check-ellipsis-match-count
+        l* depth attribute-names whole-form bound)
        {result (depth l*)
                (and/c (attribute-val/c depth identifier?)
-                      (λ (r) ((ellipsis-count/c depth) (cons r l*))))})
+                      (λ (r) ((ellipsis-count/c depth) (cons r l*))))}))
 
-  
+(define/contract/alt
+  (generate-nested-ids depth bound binder₀ format l* attribute-names whole-form)
+  generate-nested-ids-full-contract
+  (generate-nested-ids-check-ellipsis-match-count
+   l* depth attribute-names whole-form bound)
   (define (gen bottom*)
     (define v
       (let ([vs (filter-map (λ (v)
diff --git a/test/test-subtemplate.rkt b/test/test-subtemplate.rkt
index 9cddffd..8d9e972 100644
--- a/test/test-subtemplate.rkt
+++ b/test/test-subtemplate.rkt
@@ -357,7 +357,27 @@
                  (syntax-case #'([a b c] [d]) ()
                    [([xᵢ …] [pᵢ …])
                     (quasisubtemplate ([xᵢ …] [pᵢ …]))]))
-                '([a b c] [d])))
+                '([a b c] [d]))
+
+  (require (submod "../private/template-subscripts.rkt" test-private))
+  (check-exn #rx"incompatible ellipsis match counts for subscripted variables"
+             (λ ()
+               (generate-nested-ids 1
+                                    #'a
+                                    #'b
+                                    (λ (x) "fmt")
+                                    '((foo bar) (baz))
+                                    (list #'x #'y)
+                                    #'(whole))))
+  (check-equal? (map syntax-e
+                     (generate-nested-ids 1
+                                          #'a
+                                          #'b
+                                          (λ (x) "fmt")
+                                          '((foo bar) (baz quux))
+                                          (list #'x #'y)
+                                          #'(whole)))
+                '(fmt fmt)))
 
 (syntax-parse (syntax-parse #'(a b c)
                 [(xᵢ …)