diff --git a/guix/http-client.scm b/guix/http-client.scm
index 25693824ed..97a1e26d3e 100644
--- a/guix/http-client.scm
+++ b/guix/http-client.scm
@@ -243,7 +243,7 @@ (define* (http-fetch uri #:key port (text? #f) (buffered? #t)
                                                      (base64-encode
                                                       (string->utf8 str))))))
                          (_ '()))))
-      (unless buffered?
+      (unless (or buffered? (not (file-port? port)))
         (setvbuf port _IONBF))
       (let*-values (((resp data)
                      ;; Try hard to use the API du jour to get an input port.