diff --git a/src/libexpr/include/nix/expr/value.hh b/src/libexpr/include/nix/expr/value.hh
index 82db1a775..0b10b78b5 100644
--- a/src/libexpr/include/nix/expr/value.hh
+++ b/src/libexpr/include/nix/expr/value.hh
@@ -833,6 +833,7 @@ struct Value : public ValueStorage<sizeof(void *)>
 {
     friend std::string showType(const Value & v);
 
+private:
     template<InternalType... discriminator>
     bool isa() const noexcept
     {