diff --git a/src/core/chunked_stream.js b/src/core/chunked_stream.js
index 0be3140b0..26a93f673 100644
--- a/src/core/chunked_stream.js
+++ b/src/core/chunked_stream.js
@@ -98,6 +98,10 @@ class ChunkedStream {
   }
 
   ensureByte(pos) {
+    if (pos < this.progressiveDataLength) {
+      return;
+    }
+
     const chunk = Math.floor(pos / this.chunkSize);
     if (chunk === this.lastSuccessfulEnsureByteChunk) {
       return;