diff --git a/userspace/libsinsp/eventformatter.h b/userspace/libsinsp/eventformatter.h
index c3aeb797b6..f99db7eb62 100644
--- a/userspace/libsinsp/eventformatter.h
+++ b/userspace/libsinsp/eventformatter.h
@@ -71,7 +71,7 @@ class SINSP_PUBLIC sinsp_evt_formatter : public gen_event_formatter
 	// interface. It just calls resolve_tokens().
 	bool get_field_values(gen_event *evt, std::map<std::string, std::string> &fields) override;
 
-	void get_field_names(std::vector<std::string> &fields);
+	void get_field_names(std::vector<std::string> &fields) override;
 
 	gen_event_formatter::output_format get_output_format() override;
 
diff --git a/userspace/libsinsp/gen_filter.h b/userspace/libsinsp/gen_filter.h
index 3fe6e94adf..05eef13467 100644
--- a/userspace/libsinsp/gen_filter.h
+++ b/userspace/libsinsp/gen_filter.h
@@ -291,6 +291,8 @@ class gen_event_formatter
 	// (e.g. "proc.name"), to field value (e.g. "nginx")
 	virtual bool get_field_values(gen_event *evt, std::map<std::string, std::string> &fields) = 0;
 
+	virtual void get_field_names(std::vector<std::string> &fields) = 0;
+
 	virtual output_format get_output_format() = 0;
 };