Data validation may save the day of computer programmers, whatever programming language they use. In fact, processing invalid data is a waste of resources at best, and a drama at worst if the problem remains unnoticed and wrong results are used for business. Answer Set Programming is not an exception, but the quest for better and better performance resulted in systems that essentially do not validate data in any way. Even under the simplistic assumption that input and output data are eventually validated by external tools, invalid data may appear in other portions of the program, and go undetected until some other module of the designed software suddenly breaks. This paper formalizes the problem of data validation for ASP programs, introduces a declarative language to specify data validation, and presents a tool to inject data validation in ordinary programs. The proposed approach promotes fail-fast techniques at coding time without imposing any lag on the deployed system if data are pretended to be valid. Additionally, the proposed approach opens the possibility to take advantage of ASP declarativity for validating complex data of imperative programming languages.