diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index 6d86386e051a..049458a940c4 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -203,7 +203,10 @@ let
         else
           throw "unknown mirror:// site ${mirrorName}";
     in
-    if mirrorName == null then url else "${lib.head mirrorList}${lib.elemAt mirrorSplit 1}";
+    if mirrorSplit == null || mirrorName == null then
+      url
+    else
+      "${lib.head mirrorList}${lib.elemAt mirrorSplit 1}";
 in
 
 assert