diff --git a/src/libutil/abstract-setting-to-json.hh b/src/libutil/abstract-setting-to-json.hh
index d506dfb74..eea687d8a 100644
--- a/src/libutil/abstract-setting-to-json.hh
+++ b/src/libutil/abstract-setting-to-json.hh
@@ -7,7 +7,7 @@
 
 namespace nix {
 template<typename T>
-std::map<std::string, nlohmann::json> BaseSetting<T>::toJSONObject()
+std::map<std::string, nlohmann::json> BaseSetting<T>::toJSONObject() const
 {
     auto obj = AbstractSetting::toJSONObject();
     obj.emplace("value", value);
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 5b510b69e..2a5cf6212 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -221,7 +221,7 @@ nlohmann::json AbstractSetting::toJSON()
     return nlohmann::json(toJSONObject());
 }
 
-std::map<std::string, nlohmann::json> AbstractSetting::toJSONObject()
+std::map<std::string, nlohmann::json> AbstractSetting::toJSONObject() const
 {
     std::map<std::string, nlohmann::json> obj;
     obj.emplace("description", description);
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index 3f2522c38..5d7bd8e0c 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -213,7 +213,7 @@ protected:
 
     nlohmann::json toJSON();
 
-    virtual std::map<std::string, nlohmann::json> toJSONObject();
+    virtual std::map<std::string, nlohmann::json> toJSONObject() const;
 
     virtual void convertToArg(Args & args, const std::string & category);
 
@@ -306,7 +306,7 @@ public:
 
     void convertToArg(Args & args, const std::string & category) override;
 
-    std::map<std::string, nlohmann::json> toJSONObject() override;
+    std::map<std::string, nlohmann::json> toJSONObject() const override;
 };
 
 template<typename T>