diff --git a/media/engine/simulcast_encoder_adapter.cc b/media/engine/simulcast_encoder_adapter.cc
index 6f547aff89..08a463098c 100644
--- a/media/engine/simulcast_encoder_adapter.cc
+++ b/media/engine/simulcast_encoder_adapter.cc
@@ -384,16 +384,22 @@ int SimulcastEncoderAdapter::Encode(
     const uint32_t frame_timestamp_ms =
         1000 * input_image.timestamp() / 90000;  // kVideoPayloadTypeFrequency;
 
-    std::vector<VideoFrameType> stream_frame_types;
+    // If adapter is passed through and only one sw encoder does simulcast,
+    // frame types for all streams should be passed to the encoder unchanged.
+    // Otherwise a single per-encoder frame type is passed.
+    std::vector<VideoFrameType> stream_frame_types(
+        streaminfos_.size() == 1 ? NumberOfStreams(codec_) : 1);
     if (send_key_frame) {
-      stream_frame_types.push_back(VideoFrameType::kVideoFrameKey);
+      std::fill(stream_frame_types.begin(), stream_frame_types.end(),
+                VideoFrameType::kVideoFrameKey);
       streaminfos_[stream_idx].key_frame_request = false;
     } else {
       if (streaminfos_[stream_idx].framerate_controller->DropFrame(
               frame_timestamp_ms)) {
         continue;
       }
-      stream_frame_types.push_back(VideoFrameType::kVideoFrameDelta);
+      std::fill(stream_frame_types.begin(), stream_frame_types.end(),
+                VideoFrameType::kVideoFrameDelta);
     }
     streaminfos_[stream_idx].framerate_controller->AddFrame(frame_timestamp_ms);