diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 2291d618c..411364d9f 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -17,6 +17,7 @@
 #include <optional>
 #include <unordered_map>
 #include <mutex>
+#include <functional>
 
 namespace nix {
 
@@ -71,7 +72,7 @@ struct PrimOp
     /**
      * Implementation of the primop.
      */
-    PrimOpFun fun;
+    std::function<std::remove_pointer<PrimOpFun>::type> fun;
 
     /**
      * Optional experimental for this to be gated on.
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e36b800c3..77e7cf22b 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -3329,8 +3329,11 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
            callFunction. */
         /* TODO: (layus) this is absurd. An optimisation like this
            should be outside the lambda creation */
-        if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
-            return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
+        if (args[0]->isPrimOp()) {
+            auto ptr = args[0]->primOp->fun.target<decltype(&prim_lessThan)>();
+            if (ptr && *ptr == prim_lessThan)
+                return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
+        }
 
         Value * vs[] = {a, b};
         Value vBool;