diff --git a/toolkit/components/url-classifier/Classifier.cpp b/toolkit/components/url-classifier/Classifier.cpp
index d16b235c3ec6..129f20cf09cc 100644
--- a/toolkit/components/url-classifier/Classifier.cpp
+++ b/toolkit/components/url-classifier/Classifier.cpp
@@ -498,8 +498,10 @@ Classifier::Check(const nsACString& aSpec,
 
         if (LookupCache::Cast<LookupCacheV4>(cache)) {
           matchingStatistics |= PrefixMatch::eMatchV4Prefix;
+          result->mProtocolV2 = false;
         } else {
           matchingStatistics |= PrefixMatch::eMatchV2Prefix;
+          result->mProtocolV2 = true;
         }
       }
     }
diff --git a/toolkit/components/url-classifier/LookupCache.h b/toolkit/components/url-classifier/LookupCache.h
index 752c0c695d41..730bae02135e 100644
--- a/toolkit/components/url-classifier/LookupCache.h
+++ b/toolkit/components/url-classifier/LookupCache.h
@@ -26,7 +26,8 @@ namespace safebrowsing {
 class LookupResult {
 public:
   LookupResult() : mNoise(false), mProtocolConfirmed(false),
-                   mPartialHashLength(0), mConfirmed(false) {}
+                   mPartialHashLength(0), mConfirmed(false),
+                   mProtocolV2(true) {}
 
   // The fragment that matched in the LookupCache
   union {
@@ -72,6 +73,8 @@ public:
 
   // True as long as this lookup is complete and hasn't expired.
   bool mConfirmed;
+
+  bool mProtocolV2;
 };
 
 typedef nsTArray<LookupResult> LookupResultArray;
diff --git a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
index 94ef6c5958f8..346cdc42834e 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
@@ -281,8 +281,12 @@ nsUrlClassifierDBServiceWorker::DoLookup(const nsACString& spec,
   nsAutoPtr<LookupResultArray> completes(new LookupResultArray());
 
   for (uint32_t i = 0; i < results->Length(); i++) {
-    if (!mMissCache.Contains(results->ElementAt(i).hash.fixedLengthPrefix)) {
-      completes->AppendElement(results->ElementAt(i));
+    const LookupResult& lookupResult = results->ElementAt(i);
+
+    // mMissCache should only be used in V2.
+    if (!lookupResult.mProtocolV2 ||
+        !mMissCache.Contains(lookupResult.hash.fixedLengthPrefix)) {
+      completes->AppendElement(lookupResult);
     }
   }
 
@@ -1141,6 +1145,7 @@ nsUrlClassifierLookupCallback::HandleResults()
     }
   }
 
+  // TODO: Bug 1333328, Refactor cache miss mechanism for v2.
   // Some parts of this gethash request generated no hits at all.
   // Prefixes must have been removed from the database since our last update.
   // Save the prefixes we checked to prevent repeated requests
@@ -1149,7 +1154,7 @@ nsUrlClassifierLookupCallback::HandleResults()
   if (cacheMisses) {
     for (uint32_t i = 0; i < mResults->Length(); i++) {
       LookupResult &result = mResults->ElementAt(i);
-      if (!result.Confirmed() && !result.mNoise) {
+      if (result.mProtocolV2 && !result.Confirmed() && !result.mNoise) {
         cacheMisses->AppendElement(result.hash.fixedLengthPrefix);
       }
     }