diff --git a/pkgs/build-support/fetchgitlab/default.nix b/pkgs/build-support/fetchgitlab/default.nix
index 8a2b0ffa31d6..c7f2d689885f 100644
--- a/pkgs/build-support/fetchgitlab/default.nix
+++ b/pkgs/build-support/fetchgitlab/default.nix
@@ -27,9 +27,8 @@ lib.makeOverridable (
   }@args:
 
   assert (
-    lib.assertMsg (lib.xor (tag == null) (
-      rev == null
-    )) "fetchFromGitLab requires one of either `rev` or `tag` to be provided (not both)."
+    lib.xor (tag == null) (rev == null)
+    || throw "fetchFromGitLab requires one of either `rev` or `tag` to be provided (not both)."
   );
 
   let