diff --git a/lib/main.dart b/lib/main.dart
index 5bd0cd1..e4e5609 100644
--- a/lib/main.dart
+++ b/lib/main.dart
@@ -143,5 +143,5 @@ class MyCustomFormState extends StatelessWidget with InputValidationMixin {
 }
 
 mixin InputValidationMixin {
-  bool isInputValid(String value) => new RegExp(r'[0-9]+.?[0-9]*').hasMatch(value);
+  bool isInputValid(String value) => !new RegExp(r'[0-9]+.?[0-9]*').hasMatch(value);
 }