diff --git a/guix/build/union.scm b/guix/build/union.scm
index 256123c566..d46b750035 100644
--- a/guix/build/union.scm
+++ b/guix/build/union.scm
@@ -93,7 +93,7 @@ (define (resolve-collisions output dirs files)
     (cond ((null? dirs)
            ;; The inputs are all files.
            (format (current-error-port)
-                   "warning: collision encountered: ~{~a ~}~%"
+                   "~%warning: collision encountered:~%~{~a~%~}"
                    files)
 
            (let ((file (first files)))