diff --git a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
index 0270a6acdab..860093fca19 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
@@ -1430,7 +1430,7 @@ nsUrlClassifierDBService::CheckClean(const nsACString &spec,
     rv = KeyedHash(fragkey, hostprefix, prefixkey, &codedkey);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    bool found;
+    bool found = false;
     bool ready = false;  /* opportunistic probe */
     rv = mPrefixSet->Probe(codedkey, prefixkey, &ready, &found);
     NS_ENSURE_SUCCESS(rv, rv);
diff --git a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
index d7a15eea8d5..0196ac76054 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
@@ -278,6 +278,8 @@ nsUrlClassifierPrefixSet::Probe(PRUint32 aPrefix, PRUint32 aKey,
 {
   MutexAutoLock lock(mPrefixSetLock);
 
+  *aFound = false;
+
   // We might have raced here with a LoadPrefixSet call,
   // loading a saved PrefixSet with another key than the one used to probe us.
   // This must occur exactly between the GetKey call and the Probe call.