diff --git a/guix/build/utils.scm b/guix/build/utils.scm
index 2352a627e9..8e630ad586 100644
--- a/guix/build/utils.scm
+++ b/guix/build/utils.scm
@@ -9,6 +9,7 @@
 ;;; Copyright © 2020, 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2021, 2022 Maxime Devos <maximedevos@telenet.be>
 ;;; Copyright © 2021 Brendan Tildesley <mail@brendan.scot>
+;;; Copyright © 2023 Carlo Zancanaro <carlo@zancanaro.id.au>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -729,18 +730,22 @@ (define (every* pred lst)
 (define* (alist-cons-before reference key value alist
                             #:optional (key=? equal?))
   "Insert the KEY/VALUE pair before the first occurrence of a pair whose key
-is REFERENCE in ALIST.  Use KEY=? to compare keys."
+is REFERENCE in ALIST.  Use KEY=? to compare keys.  An error is raised when no
+such pair exists."
   (let-values (((before after)
                 (break (match-lambda
                         ((k . _)
                          (key=? k reference)))
                        alist)))
-    (append before (alist-cons key value after))))
+    (match after
+      ((_ _ ...)
+       (append before (alist-cons key value after))))))
 
 (define* (alist-cons-after reference key value alist
                            #:optional (key=? equal?))
   "Insert the KEY/VALUE pair after the first occurrence of a pair whose key
-is REFERENCE in ALIST.  Use KEY=? to compare keys."
+is REFERENCE in ALIST.  Use KEY=? to compare keys.  An error is raised when
+no such pair exists."
   (let-values (((before after)
                 (break (match-lambda
                         ((k . _)
@@ -748,9 +753,7 @@ (define* (alist-cons-after reference key value alist
                        alist)))
     (match after
       ((reference after ...)
-       (append before (cons* reference `(,key . ,value) after)))
-      (()
-       (append before `((,key . ,value)))))))
+       (append before (cons* reference `(,key . ,value) after))))))
 
 (define* (alist-replace key value alist #:optional (key=? equal?))
   "Replace the first pair in ALIST whose car is KEY with the KEY/VALUE pair.
diff --git a/tests/build-utils.scm b/tests/build-utils.scm
index 7f4f12ccc7..3babf5d544 100644
--- a/tests/build-utils.scm
+++ b/tests/build-utils.scm
@@ -41,17 +41,17 @@ (define-module (test build-utils)
   '((a . 1) (x . 42) (b . 2) (c . 3))
   (alist-cons-before 'b 'x 42 '((a . 1) (b . 2) (c . 3))))
 
-(test-equal "alist-cons-before, reference not found"
-  '((a . 1) (b . 2) (c . 3) (x . 42))
-  (alist-cons-before 'z 'x 42 '((a . 1) (b . 2) (c . 3))))
+(test-assert "alist-cons-before, reference not found"
+  (not (false-if-exception
+        (alist-cons-before 'z 'x 42 '((a . 1) (b . 2) (c . 3))))))
 
 (test-equal "alist-cons-after"
   '((a . 1) (b . 2) (x . 42) (c . 3))
   (alist-cons-after 'b 'x 42 '((a . 1) (b . 2) (c . 3))))
 
-(test-equal "alist-cons-after, reference not found"
-  '((a . 1) (b . 2) (c . 3) (x . 42))
-  (alist-cons-after 'z 'x 42 '((a . 1) (b . 2) (c . 3))))
+(test-assert "alist-cons-after, reference not found"
+  (not (false-if-exception
+        (alist-cons-after 'z 'x 42 '((a . 1) (b . 2) (c . 3))))))
 
 (test-equal "alist-replace"
   '((a . 1) (b . 77) (c . 3))