diff --git a/xpcom/string/public/nsTSubstring.h b/xpcom/string/public/nsTSubstring.h
index 14bba499d29d..fb8bc841a4cb 100644
--- a/xpcom/string/public/nsTSubstring.h
+++ b/xpcom/string/public/nsTSubstring.h
@@ -659,8 +659,25 @@ class nsTSubstring_CharT
          * this function returns false if is unable to allocate sufficient
          * memory.
          */
-      PRBool NS_FASTCALL ReplacePrep( index_type cutStart, size_type cutLength, size_type newLength );
+      PRBool ReplacePrep(index_type cutStart, size_type cutLength,
+                         size_type newLength)
+      {
+        cutLength = NS_MIN(cutLength, mLength - cutStart);
+        PRUint32 newTotalLen = mLength - cutLength + newLength;
+        if (cutStart == mLength && Capacity() > newTotalLen) {
+          mFlags &= ~F_VOIDED;
+          mData[newTotalLen] = char_type(0);
+          mLength = newTotalLen;
+          return PR_TRUE;
+        }
+        return ReplacePrepInternal(cutStart, cutLength, newLength, newTotalLen);
+      }
 
+      PRBool NS_FASTCALL ReplacePrepInternal(index_type cutStart,
+                                             size_type cutLength,
+                                             size_type newFragLength,
+                                             size_type newTotalLength);
+      
         /**
          * returns the number of writable storage units starting at mData.
          * the value does not include space for the null-terminator character.
diff --git a/xpcom/string/src/nsTSubstring.cpp b/xpcom/string/src/nsTSubstring.cpp
index 2510c71c2d9d..1f31013e6c21 100644
--- a/xpcom/string/src/nsTSubstring.cpp
+++ b/xpcom/string/src/nsTSubstring.cpp
@@ -188,13 +188,9 @@ nsTSubstring_CharT::Finalize()
   }
 
 PRBool
-nsTSubstring_CharT::ReplacePrep( index_type cutStart, size_type cutLen, size_type fragLen )
+nsTSubstring_CharT::ReplacePrepInternal(index_type cutStart, size_type cutLen,
+                                        size_type fragLen, size_type newLen)
   {
-    // bound cut length
-    cutLen = NS_MIN(cutLen, mLength - cutStart);
-
-    PRUint32 newLen = mLength - cutLen + fragLen;
-
     char_type* oldData;
     PRUint32 oldFlags;
     if (!MutatePrep(newLen, &oldData, &oldFlags))