diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index 804974954d10..2c802bb3679c 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -85,8 +85,7 @@ in
 }:
 
 assert builtins.isList urls;
-assert urls != [] -> url == "";
-assert url != "" -> urls == [];
+assert urls == [] || url == "";
 
 
 let