diff --git a/pkgs/by-name/xr/xreader/package.nix b/pkgs/by-name/xr/xreader/package.nix
index 78d0c4d09588..6cbe4aa32ac7 100644
--- a/pkgs/by-name/xr/xreader/package.nix
+++ b/pkgs/by-name/xr/xreader/package.nix
@@ -57,6 +57,7 @@ stdenv.mkDerivation rec {
   ];
 
   mesonFlags = [
+    # FIXME: `MathJax.js` is only available in MathJax 2.7.x.
     "-Dmathjax-directory=${mathjax}"
     "-Dintrospection=true"
   ]