addCondition:labelled does not seem to be working the way I expected.

I just want to validate that a given text field is not empty

I have a description definition method and setter:

descriptionTags
^MAStringDescription new
    selectorAccessor: #tagsConverter;
    label: 'Tags';
    priority: 40;
    addCondition: [:value | value notEmpty ]
        labelled: 'Please provide at least one tag';
    yourself

tagsConverter: aString
    tags := aString asUppercase subStrings asSet

On submit, I get an error that nil does not understand asUppercase. Shouldn't the condition fail before the setter is called? I've tried [ :value | value notNil ], and beRequired as well, got the same error. I also tried to change the initialize method of the object I'm describing to set tags to an empty collection but that didn't work either.

Thanks,
Joel