diff --git a/src/formula.hpp b/src/formula.hpp
index 13adfbbc9f1..5566ac7e3f2 100644
--- a/src/formula.hpp
+++ b/src/formula.hpp
@@ -50,7 +50,7 @@ public:
 	const std::string& str() const { return str_; }
 
 private:
-	formula() {}
+	formula();
 	expression_ptr expr_;
 	std::string str_;
 };