Index: lib/filterClasses.js
===================================================================
--- a/lib/filterClasses.js
+++ b/lib/filterClasses.js
@@ -477,54 +477,58 @@
     // upper-case to avoid false positives here. Instead we need to
     // change the way filter options are parsed.
     if (this.sitekeys &&
         (!sitekey || this.sitekeys.indexOf(sitekey.toUpperCase()) < 0))
     {
       return false;
     }
 
+    let {domains} = this;
+
     // If no domains are set the rule matches everywhere
-    if (!this.domains)
+    if (!domains)
       return true;
 
     // If the document has no host name, match only if the filter
     // isn't restricted to specific domains
     if (!docDomain)
-      return this.domains.get("");
+      return domains.get("");
 
     docDomain = docDomain.replace(/\.+$/, "").toLowerCase();
 
     while (true)
     {
-      let isDomainIncluded = this.domains.get(docDomain);
+      let isDomainIncluded = domains.get(docDomain);
       if (typeof isDomainIncluded != "undefined")
         return isDomainIncluded;
 
       let nextDot = docDomain.indexOf(".");
       if (nextDot < 0)
         break;
       docDomain = docDomain.substr(nextDot + 1);
     }
-    return this.domains.get("");
+    return domains.get("");
   },
 
   /**
    * Checks whether this filter is active only on a domain and its subdomains.
    * @param {string} docDomain
    * @return {boolean}
    */
   isActiveOnlyOnDomain(docDomain)
   {
-    if (!docDomain || !this.domains || this.domains.get(""))
+    let {domains} = this;
+
+    if (!docDomain || !domains || domains.get(""))
       return false;
 
     docDomain = docDomain.replace(/\.+$/, "").toLowerCase();
 
-    for (let [domain, isIncluded] of this.domains)
+    for (let [domain, isIncluded] of domains)
     {
       if (isIncluded && domain != docDomain)
       {
         if (domain.length <= docDomain.length)
           return false;
 
         if (!domain.endsWith("." + docDomain))
           return false;
@@ -535,18 +539,19 @@
   },
 
   /**
    * Checks whether this filter is generic or specific
    * @return {boolean}
    */
   isGeneric()
   {
-    return !(this.sitekeys && this.sitekeys.length) &&
-            (!this.domains || this.domains.get(""));
+    let {sitekeys, domains} = this;
+
+    return !(sitekeys && sitekeys.length) && (!domains || domains.get(""));
   },
 
   /**
    * See Filter.serialize()
    * @inheritdoc
    */
   serialize(buffer)
   {