diff --git a/lib/streams.js b/lib/streams.js
index 83c1cc1..8305c71 100644
--- a/lib/streams.js
+++ b/lib/streams.js
@@ -173,7 +173,7 @@ class BlockMapReadStream extends Readable {
 
   _prefetch(index) {
     const blocks = []
-    for (; index < this._endIndex; index++) blocks.push(this._blockMap.blocks[index])
+    for (; index < this._endIndex; index++) blocks.push(this._blockMap.blocks[index].index)
     this._range = this.core.download({ blocks })
   }