diff --git a/Foundation/include/Poco/Dynamic/Struct.h b/Foundation/include/Poco/Dynamic/Struct.h
index d51011524..051d82c90 100644
--- a/Foundation/include/Poco/Dynamic/Struct.h
+++ b/Foundation/include/Poco/Dynamic/Struct.h
@@ -191,7 +191,7 @@ public:
 		return keys;
 	}
 
-	std::string toString()
+	std::string toString() const
 	{
 		std::string str;
 		Var(*this).convert<std::string>(str);