diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index 0777aa3d3e..ba2b2d2d4e 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -537,6 +537,7 @@ (define batch
           (()
            (match (drop requests processed)
              (()
+              (close-port p)
               (reverse result))
              (remainder
               (connect p remainder result))))