diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a62cee299..5e2f71649 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -937,6 +937,9 @@ ListBuilder::ListBuilder(EvalState & state, size_t size)
     state.nrListElems += size;
 }
 
+Value * EvalState::getBool(bool b) {
+    return b ? &vTrue : &vFalse;
+}
 
 unsigned long nrThunks = 0;
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index eac83fe34..f15d19653 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -650,6 +650,11 @@ public:
         return ListBuilder(*this, size);
     }
 
+    /**
+     * Return a boolean `Value *` without allocating.
+     */
+    Value *getBool(bool b);
+
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 0f2aaa83f..d0fcfd194 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2845,8 +2845,7 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg
 
     auto attrs = state.buildBindings(args[0]->lambda.fun->formals->formals.size());
     for (auto & i : args[0]->lambda.fun->formals->formals)
-        // !!! should optimise booleans (allocate only once)
-        attrs.alloc(i.name, i.pos).mkBool(i.def);
+        attrs.insert(i.name, state.getBool(i.def), i.pos);
     v.mkAttrs(attrs);
 }