diff --git a/lib/extract-class-names.js b/lib/extract-class-names.js
index 71e88b6..517be09 100644
--- a/lib/extract-class-names.js
+++ b/lib/extract-class-names.js
@@ -42,11 +42,14 @@ module.exports = function (cssSelector) {
     return []
   }
 
+  // handling empty inputs
+  if (!result) {
+    return []
+  }
+
   var classNames = []
 
-  if (result.type === 'rule') {
-    classNames = extractClassNames(result.rule)
-  } else if (result.type === 'ruleSet') {
+  if (result.type === 'ruleSet') {
     var rule = result.rule
     while (rule) {
       classNames.push.apply(classNames, extractClassNames(rule))
diff --git a/test/rules/no-bootstrap-classes.js b/test/rules/no-bootstrap-classes.js
index f519ba8..3f154e8 100644
--- a/test/rules/no-bootstrap-classes.js
+++ b/test/rules/no-bootstrap-classes.js
@@ -26,7 +26,9 @@ eslintTester.run('no-bootstrap-classes', rule, {
     '$("p::first-line");',
     '$("invalid::selector()");',
     '$("a[href^=/], .container:has(nav) > a[href]:lt($var)");',
-    '$$("//somethinginvalid[@attr=\'test\']")'
+    '$$("//somethinginvalid[@attr=\'test\']")',
+    'element(by.css("a[href]"));',
+    '$("");'
   ],
 
   invalid: [