--- a/xpcom/io/nsStreamUtils.cpp
+++ b/xpcom/io/nsStreamUtils.cpp
@@ -739,17 +739,18 @@ TestInputStream(nsIInputStream* aInStr,
                 void* aClosure,
                 const char* aBuffer,
                 uint32_t aOffset,
                 uint32_t aCount,
                 uint32_t* aCountWritten)
 {
   bool* result = static_cast<bool*>(aClosure);
   *result = true;
-  return NS_ERROR_ABORT;  // don't call me anymore
+  *aCountWritten = 0;
+  return NS_ERROR_ABORT; // don't call me anymore
 }
 
 bool
 NS_InputStreamIsBuffered(nsIInputStream* aStream)
 {
   nsCOMPtr<nsIBufferedInputStream> bufferedIn = do_QueryInterface(aStream);
   if (bufferedIn) {
     return true;
@@ -766,17 +767,18 @@ TestOutputStream(nsIOutputStream* aOutSt
                  void* aClosure,
                  char* aBuffer,
                  uint32_t aOffset,
                  uint32_t aCount,
                  uint32_t* aCountRead)
 {
   bool* result = static_cast<bool*>(aClosure);
   *result = true;
-  return NS_ERROR_ABORT;  // don't call me anymore
+  *aCountRead = 0;
+  return NS_ERROR_ABORT; // don't call me anymore
 }
 
 bool
 NS_OutputStreamIsBuffered(nsIOutputStream* aStream)
 {
   nsCOMPtr<nsIBufferedOutputStream> bufferedOut = do_QueryInterface(aStream);
   if (bufferedOut) {
     return true;