--- a/gfx/thebes/gfxUtils.cpp
+++ b/gfx/thebes/gfxUtils.cpp
@@ -1194,16 +1194,19 @@ gfxUtils::EncodeSourceSurface(SourceSurf
   if (!imgData.initCapacity(bufSize)) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
   uint32_t numReadThisTime = 0;
   while ((rv = imgStream->Read(imgData.begin() + imgSize,
                                bufSize - imgSize,
                                &numReadThisTime)) == NS_OK && numReadThisTime > 0)
   {
+    // Update the length of the vector without overwriting the new data.
+    imgData.growByUninitialized(numReadThisTime);
+
     imgSize += numReadThisTime;
     if (imgSize == bufSize) {
       // need a bigger buffer, just double
       bufSize *= 2;
       if (!imgData.resizeUninitialized(bufSize)) {
         return NS_ERROR_OUT_OF_MEMORY;
       }
     }