diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index 2c802bb3679..d4b46cf9e9d 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -85,7 +85,7 @@ in
 }:
 
 assert builtins.isList urls;
-assert urls == [] || url == "";
+assert (urls == []) != (url == "");
 
 
 let