diff --git a/src/Middleware/SpaServices.Extensions/src/Util/EventedStreamReader.cs b/src/Middleware/SpaServices.Extensions/src/Util/EventedStreamReader.cs
index aafd630853ce..8cb33787a544 100644
--- a/src/Middleware/SpaServices.Extensions/src/Util/EventedStreamReader.cs
+++ b/src/Middleware/SpaServices.Extensions/src/Util/EventedStreamReader.cs
@@ -111,7 +111,7 @@ private async Task Run()
                 // get the rest
                 if (lineBreakPos < 0 && startPos < chunkLength)
                 {
-                    _linesBuffer.Append(buf, startPos, chunkLength);
+                    _linesBuffer.Append(buf, startPos, chunkLength - startPos);
                 }
             }
         }