diff --git a/toolkit/components/url-classifier/LookupCache.h b/toolkit/components/url-classifier/LookupCache.h
index 139bb4cc47a9..5bca76d08178 100644
--- a/toolkit/components/url-classifier/LookupCache.h
+++ b/toolkit/components/url-classifier/LookupCache.h
@@ -44,7 +44,13 @@ public:
 
   nsCString PartialHash() {
     MOZ_ASSERT(mPartialHashLength <= COMPLETE_SIZE);
-    return nsCString(reinterpret_cast<char*>(hash.complete.buf), mPartialHashLength);
+    if (mNoise) {
+      return nsCString(reinterpret_cast<char*>(hash.fixedLengthPrefix.buf),
+                       PREFIX_SIZE);
+    } else {
+      return nsCString(reinterpret_cast<char*>(hash.complete.buf),
+                       mPartialHashLength);
+    }
   }
 
   nsCString PartialHashHex() {
diff --git a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
index 9a395eba5090..6481469c40e6 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
@@ -1097,10 +1097,7 @@ nsUrlClassifierLookupCallback::LookupComplete(nsTArray<LookupResult>* results)
 
         // Bug 1323953 - Send the first 4 bytes for completion no matter how
         // long we matched the prefix.
-        nsAutoCString partialHash;
-        partialHash.Assign(reinterpret_cast<char*>(&result.hash.fixedLengthPrefix),
-                           PREFIX_SIZE);
-        nsresult rv = completer->Complete(partialHash,
+        nsresult rv = completer->Complete(result.PartialHash(),
                                           gethashUrl,
                                           result.mTableName,
                                           this);