Customizing calculated value but get NaN as result

Hi, I'm trying to get a calculated value on a custom attribute, this attribute is an integer type, and is also integer kind on presentation. I want it to count each time it reaches to an specific state. I'm using the following script:

dojo.provide("counterString");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
(function() {
var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
dojo.declare("counterString", null, {
    getValue: function(attributeId, workItem, configuration) {
    var counter = parseInt(workItem.getValue("counterString"));
    console.log("Counter: " + counter + "\n\n");
        var stat = workItem.getValue(WorkItemAttributes.STATE);
        console.log(counter);
        console.log(stat);
        if(stat === "s5")
            {  
                console.log("Actual value:" + counter);
                counter = counter+1;
                console.log("New Value:" + counter);
            }
                return parseInt(counter);
    }
});
})();

The state condition works fine, but not for the counter, when I check my log it says "Counter: NaN".

What should I change on my script?