diff --git a/guix/build/utils.scm b/guix/build/utils.scm
index 305ce7d4ee..1808c63ce5 100644
--- a/guix/build/utils.scm
+++ b/guix/build/utils.scm
@@ -170,6 +170,7 @@ (define (substitute file pattern match-proc)
                            (display line out)
                            (newline out)))
                     (loop (read-line in)))))))
+        (close out)
         (rename-file template file))
       (lambda (key . args)
         (false-if-exception (delete-file template))))))