diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 48598acaf..27d8ddf35 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -153,6 +153,7 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
     fetchTree(state, pos, args, v, std::nullopt);
 }
 
+// FIXME: document
 static RegisterPrimOp primop_fetchTree("fetchTree", 1, prim_fetchTree);
 
 static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,