diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index c8d83814f..b470fa315 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -78,5 +78,15 @@ inline void EvalState::forceList(Value & v, const Pos & pos)
         throwTypeError("value is %1% while a list was expected, at %2%", v, pos);
 }
 
+inline bool EvalState::isFunctor(Value& fun)
+{
+    if (fun.type == tAttrs) {
+      auto found = fun.attrs->find(sFunctor);
+      if (found != fun.attrs->end()) {
+        return true;
+      }
+    }
+    return false;
+}
 
 }
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 74f88e498..93c3111af 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1294,7 +1294,7 @@ bool EvalState::forceBool(Value & v)
 void EvalState::forceFunction(Value & v, const Pos & pos)
 {
     forceValue(v);
-    if (v.type != tLambda && v.type != tPrimOp && v.type != tPrimOpApp)
+    if (v.type != tLambda && v.type != tPrimOp && v.type != tPrimOpApp && !isFunctor(v))
         throwTypeError("value is %1% while a function was expected, at %2%", v, pos);
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 8df0084fd..eb55f6d4d 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -213,6 +213,8 @@ public:
        elements and attributes are compared recursively. */
     bool eqValues(Value & v1, Value & v2);
 
+    bool isFunctor(Value & fun);
+
     void callFunction(Value & fun, Value & arg, Value & v, const Pos & pos);
     void callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos);