--- a/mfbt/BufferList.h
+++ b/mfbt/BufferList.h
@@ -392,16 +392,17 @@ BufferList<AllocPolicy>::MoveFallible(bo
   BufferList<OtherAllocPolicy> result(0, 0, mStandardCapacity, aAP);
 
   IterImpl iter = Iter();
   while (!iter.Done()) {
     size_t toAdvance = iter.RemainingInSegment();
 
     if (!toAdvance || !result.mSegments.append(typename BufferList<OtherAllocPolicy>::Segment(iter.mData, toAdvance, toAdvance))) {
       *aSuccess = false;
+      result.mSegments.clear();
       return result;
     }
     iter.Advance(*this, toAdvance);
   }
 
   result.mSize = mSize;
   mSegments.clear();
   mSize = 0;