--- a/toolkit/components/url-classifier/LookupCache.h
+++ b/toolkit/components/url-classifier/LookupCache.h
@@ -123,17 +123,17 @@ public:
   virtual nsresult Open();
   virtual nsresult Init() = 0;
   virtual nsresult ClearPrefixes() = 0;
   virtual nsresult Has(const Completion& aCompletion,
                        bool* aHas, bool* aComplete) = 0;
 
   template<typename T>
   static T* Cast(LookupCache* aThat) {
-    return (T::VER == aThat->Ver() ? reinterpret_cast<T*>(aThat) : nullptr);
+    return ((aThat && T::VER == aThat->Ver()) ? reinterpret_cast<T*>(aThat) : nullptr);
   }
 
 private:
   nsresult Reset();
   nsresult LoadPrefixSet();
 
   virtual nsresult StoreToFile(nsIFile* aFile) = 0;
   virtual nsresult LoadFromFile(nsIFile* aFile) = 0;