diff --git a/util/aligned_buffer.h b/util/aligned_buffer.h
index 0024e4df4..2201b4877 100644
--- a/util/aligned_buffer.h
+++ b/util/aligned_buffer.h
@@ -121,6 +121,7 @@ public:
         ~static_cast<uintptr_t>(alignment_ - 1));
 
     if (copy_data) {
+      assert(bufstart_ + copy_offset + copy_len <= bufstart_ + cursize_);
       memcpy(new_bufstart, bufstart_ + copy_offset, copy_len);
       cursize_ = copy_len;
     } else {
diff --git a/util/file_reader_writer.cc b/util/file_reader_writer.cc
index 127a19475..e8b03af49 100644
--- a/util/file_reader_writer.cc
+++ b/util/file_reader_writer.cc
@@ -672,7 +672,14 @@ Status FilePrefetchBuffer::Prefetch(RandomAccessFileReader* reader,
       chunk_len = buffer_len_ - chunk_offset_in_buffer;
       assert(chunk_offset_in_buffer % alignment == 0);
       assert(chunk_len % alignment == 0);
-      copy_data_to_new_buffer = true;
+      assert(chunk_offset_in_buffer + chunk_len <=
+             buffer_offset_ + buffer_len_);
+      if (chunk_len > 0) {
+        copy_data_to_new_buffer = true;
+      } else {
+        // this reset is not necessary, but just to be safe.
+        chunk_offset_in_buffer = 0;
+      }
     }
   }