diff --git a/guix/nar.scm b/guix/nar.scm
index 4bc2deb229..89a71302e0 100644
--- a/guix/nar.scm
+++ b/guix/nar.scm
@@ -112,7 +112,8 @@
   (write-long-long size p)
   (call-with-binary-input-file file
     ;; Use `sendfile' when available (Guile 2.0.8+).
-    (if (compile-time-value (defined? 'sendfile))
+    (if (and (compile-time-value (defined? 'sendfile))
+             (file-port? p))
         (cut sendfile p <> size 0)
         (cut dump <> p size)))
   (write-padding size p))
diff --git a/tests/nar.scm b/tests/nar.scm
index 9f21f990c8..7ae8cf0aa7 100644
--- a/tests/nar.scm
+++ b/tests/nar.scm
@@ -183,6 +183,13 @@
 
 (test-begin "nar")
 
+(test-assert "write-file supports non-file output ports"
+  (let ((input  (string-append (dirname (search-path %load-path "guix.scm"))
+                               "/guix"))
+        (output (%make-void-port "w")))
+    (write-file input output)
+    #t))
+
 (test-assert "write-file + restore-file"
   (let* ((input  (string-append (dirname (search-path %load-path "guix.scm"))
                                 "/guix"))