diff --git a/file/file_prefetch_buffer.cc b/file/file_prefetch_buffer.cc
index 764f450c03f..315ee732682 100644
--- a/file/file_prefetch_buffer.cc
+++ b/file/file_prefetch_buffer.cc
@@ -856,7 +856,9 @@ bool FilePrefetchBuffer::TryReadFromCacheUntracked(
   if (copy_to_overlap_buffer) {
     buf = overlap_buf_;
   }
+  assert(buf->offset_ <= offset);
   uint64_t offset_in_buffer = offset - buf->offset_;
+  assert(n <= buf->CurrentSize() - offset_in_buffer);
   *result = Slice(buf->buffer_.BufferStart() + offset_in_buffer, n);
   if (prefetched) {
     readahead_size_ = std::min(max_readahead_size_, readahead_size_ * 2);