--- a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
@@ -151,19 +151,26 @@ nsUrlClassifierPrefixSet::GetPrefixesNat
   }
 
   uint32_t prefixIdxLength = mIndexPrefixes.Length();
   uint32_t prefixCnt = 0;
 
   for (uint32_t i = 0; i < prefixIdxLength; i++) {
     uint32_t prefix = mIndexPrefixes[i];
 
+    if (prefixCnt >= mTotalPrefixes) {
+      return NS_ERROR_FAILURE;
+    }
     outArray[prefixCnt++] = prefix;
+
     for (uint32_t j = 0; j < mIndexDeltas[i].Length(); j++) {
       prefix += mIndexDeltas[i][j];
+      if (prefixCnt >= mTotalPrefixes) {
+        return NS_ERROR_FAILURE;
+      }
       outArray[prefixCnt++] = prefix;
     }
   }
 
   NS_ASSERTION(mTotalPrefixes == prefixCnt, "Lengths are inconsistent");
   return NS_OK;
 }