Clarify error message for NaN

index.js

@@ -9,6 +9,6 @@ module.exports = function (value) {
 	if (!isNumberObject(value)) {
 		throw new ValidationError("Must be a number");
 	} else if (Number.isNaN(value)) {
-		throw new ValidationError(`Must not be NaN`);		
+		throw new ValidationError(`Must be a number (must not be NaN)`);
 	}
 };