Add error code

index.js

@@ -5,6 +5,6 @@ const isString = require("is-string");
 
 module.exports = function (value) {
 	if (!isString(value)) {
-		return new ValidationError("Must be a string");
+		return new ValidationError("Must be a string", { code: "validatem.is-string" });
 	}
 };