Cannot add new mail filter to empty list of filters.

When clicking on (+) to add condition script fails in the function below with error "filter is undefined" on the line: "if (!filter.rules)"

function onRuleAddClick(event) {
var filterRules = $("filterRules");
if (filterRules) {
var newRule = createFilterRule();
if (!filter.rules)
filter.rules = [];
filter.rules.push(newRule);
var newRuleDiv = appendRule(filterRules, newRule);
setSelectedRuleDiv(newRuleDiv);
filterRules.scrollTop = newRuleDiv.offsetTop;
}
Event.stop(event);
}