Index: lib/filterClasses.js
===================================================================
--- a/lib/filterClasses.js
+++ b/lib/filterClasses.js
@@ -456,21 +456,20 @@
           if (!domains)
             domains = new Map();
 
           domains.set(domain, include);
         }
         if (domains)
           domains.set("", !hasIncludes);
       }
-
-      this.domainSource = null;
     }
 
     Object.defineProperty(this, "domains", {value: domains, enumerable: true});
+    delete this.domainSource;
     return this.domains;
   },
 
   /**
    * Array containing public keys of websites that this filter should apply to
    * @type {?string[]}
    */
   sitekeys: null,
@@ -706,24 +705,22 @@
     // several times on Safari, due to WebKit bug 132872
     let prop = Object.getOwnPropertyDescriptor(this, "sitekeys");
     if (prop)
       return prop.value;
 
     let sitekeys = null;
 
     if (this.sitekeySource)
-    {
       sitekeys = this.sitekeySource.split("|");
-      this.sitekeySource = null;
-    }
 
     Object.defineProperty(
       this, "sitekeys", {value: sitekeys, enumerable: true}
     );
+    delete this.sitekeySource;
     return this.sitekeys;
   },
 
   /**
    * Tests whether the URL matches this filter
    * @param {string} location URL to be tested
    * @param {number} typeMask bitmask of content / request types to match
    * @param {string} [docDomain] domain name of the document that loads the URL