diff --git a/mozilla/xpcom/string/src/nsStringObsolete.cpp b/mozilla/xpcom/string/src/nsStringObsolete.cpp
index a3d55db8bf6..e6c6c849b20 100644
--- a/mozilla/xpcom/string/src/nsStringObsolete.cpp
+++ b/mozilla/xpcom/string/src/nsStringObsolete.cpp
@@ -780,7 +780,7 @@ RFindCharInSet( const CharT* data, PRUint32 dataLen, const SetCharT* set )
   {
     CharT filter = nsBufferRoutines<CharT>::get_find_in_set_filter(set);
 
-    for (const CharT* iter = data + dataLen; iter >= data; --iter)
+    for (const CharT* iter = data + dataLen - 1; iter >= data; --iter)
       {
         CharT currentChar = *iter;
         if (currentChar & filter)
diff --git a/mozilla/xpcom/string/src/nsTStringObsolete.cpp b/mozilla/xpcom/string/src/nsTStringObsolete.cpp
index 96d10d46f70..5ed065ad2e8 100644
--- a/mozilla/xpcom/string/src/nsTStringObsolete.cpp
+++ b/mozilla/xpcom/string/src/nsTStringObsolete.cpp
@@ -127,15 +127,13 @@ nsTString_CharT::FindCharInSet( const char* aSet, PRInt32 aOffset ) const
 PRInt32
 nsTString_CharT::RFindCharInSet( const CharT* aSet, PRInt32 aOffset ) const
   {
-    if (aOffset < 0)
-      aOffset = 0;
-    else if (aOffset >= PRInt32(mLength))
-      return kNotFound;
-    
-    PRInt32 result = ::RFindCharInSet(mData + aOffset, mLength - aOffset, aSet);
-    if (result != kNotFound)
-      result += aOffset;
-    return result;
+    // We want to pass a "data length" to ::RFindCharInSet
+    if (aOffset < 0 || aOffset > PRInt32(mLength))
+      aOffset = mLength;
+    else
+      ++aOffset;
+
+    return ::RFindCharInSet(mData, aOffset, aSet);
   }