diff --git a/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc b/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc
index 78d73969c8fa8..386aa2f5d6550 100644
--- a/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc
+++ b/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc
@@ -2069,11 +2069,11 @@ HRESULT MediaFoundationVideoEncodeAccelerator::PopulateInputSampleBuffer(
                                            : input_stream_info.cbAlignment - 1,
         &input_buffer);
     RETURN_ON_HR_FAILURE(hr, "Failed to create memory buffer", hr);
+    // Only AllocationSize(NV12, input_visible_size_) bytes are written below by
+    // ConvertAndScale(); advertise that, not cbSize, so the MFT is not handed
+    // an uninitialised [AllocationSize, cbSize) tail as valid data.
     hr = input_buffer->SetCurrentLength(
-        input_stream_info.cbSize
-            ? input_stream_info.cbSize
-            : VideoFrame::AllocationSize(kTargetPixelFormat,
-                                         input_visible_size_));
+        VideoFrame::AllocationSize(kTargetPixelFormat, input_visible_size_));
     RETURN_ON_HR_FAILURE(hr, "Failed to set length on buffer", hr);
     hr = input_sample->AddBuffer(input_buffer.Get());
     RETURN_ON_HR_FAILURE(hr, "Failed to add buffer to sample", hr);