diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index c74131a9e60..0fcbf2f3d0d 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -81,16 +81,16 @@ assert builtins.isList urls;
 assert urls != [] -> url == "";
 assert url != "" -> urls == [];
 
-assert showURLs || (outputHash != "" && outputHashAlgo != "")
-    || md5 != "" || sha1 != "" || sha256 != "";
 
 let
 
+  hasHash = showURLs || (outputHash != "" && outputHashAlgo != "")
+    || md5 != "" || sha1 != "" || sha256 != "";
   urls_ = if urls != [] then urls else [url];
 
 in
 
-stdenv.mkDerivation {
+if (!hasHash) then throw "Specify hash for fetchurl fixed-output derivation: ${stdenv.lib.concatStringsSep ", " urls_}" else stdenv.mkDerivation {
   name =
     if showURLs then "urls"
     else if name != "" then name