Validation on a BlockType - no errormessage in UI when saving block

We have a block with properties Image and ImageAlt. To get the ImageAlt to be required if the user add an Image we created a custom validation class for the block.

public class ImageBlockValidator : IValidate<ImageBlock>
    {
        public IEnumerable<ValidationError> Validate(ImageBlock instance)
        {
            if (instance.Image != null && string.IsNullOrEmpty(instance.ImageAlt))
            {
                return new[] { new ValidationError
                {
                    ErrorMessage = LocalizationService.Current.TranslateKey("/validationmessages/imageblock/imagealt/heading"),
                    PropertyName = instance.GetPropertyName(block => block.ImageAlt),
                    RelatedProperties = new[] { "Image" }, 
                    Severity = ValidationErrorSeverity.Error,
                    ValidationType = ValidationErrorType.StorageValidation
                }};
            }
            return new ValidationError[0];
        }
    }

But when a user add an Image and click "Spara" no error message is shown i the UI, the "Spara" button gets inactive and nothing happens. The block isn´t saved.

But if the user saves the block before adding an Image and then goes back to edit the block and adds an Image the validation error message is shown.