diff --git a/guix/build/download.scm b/guix/build/download.scm
index 65d18eb839..ae59b0109c 100644
--- a/guix/build/download.scm
+++ b/guix/build/download.scm
@@ -231,7 +231,8 @@ host name without trailing dot."
                                (resolve-interface '(web client))
                                'current-http-proxy))
                          (parameterize ((current-http-proxy #f))
-                           (when (getenv "https_proxy")
+                           (when (and=> (getenv "https_proxy")
+                                        (negate string-null?))
                              (format (current-error-port)
                                      "warning: 'https_proxy' is ignored~%"))
                            (thunk))