diff --git a/pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl b/pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl
index 21e61bc54d0e..8bed3fc6d09c 100755
--- a/pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl
+++ b/pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl
@@ -307,7 +307,7 @@ foreach my $pkg (sort (keys %pkgURLs)) {
     name = "$pkgNames{$pkg}";
     builder = ./builder.sh;
     src = fetchurl {
-      url = $pkgURLs{$pkg};
+      url = "$pkgURLs{$pkg}";
       sha256 = "$pkgHashes{$pkg}";
     };
     hardeningDisable = [ "bindnow" "relro" ];