[1.5 version] Multi-input FFmpegFrameFilter initialization error：avfilter_graph_config() error -22

I wanted to mix two audios into a single one, I used the FFmpegFrameFilter to do this.

my javacv version:

javaCVVersion := "1.5" javaCppVersion := "1.5"

here's my code:

val filterStr = "amix=inputs=2"
val filter = new FFmpegFrameFilter(filterStr, 2)
filter.start()

when start() executed, an exception occurred:

org.bytedeco.javacv.FrameFilter$Exception: avfilter_graph_config() error -22 at org.bytedeco.javacv.FFmpegFrameFilter.startAudioUnsafe(FFmpegFrameFilter.java:464) at org.bytedeco.javacv.FFmpegFrameFilter.startUnsafe(FFmpegFrameFilter.java:271) at org.bytedeco.javacv.FFmpegFrameFilter.start(FFmpegFrameFilter.java:251)

But when I change the filterStr to "amix=inputs=1", the input becomes a single one, the exception won't occur.

I tried some other filter strings, and I found single input works well, but when it comes to multiple input situations, the filter can't start successfully, and this exception occurs.

Btw, I mixed two mp3 audios successfully using ffmpeg command line with the same filter aforementioned.

Any Initialization steps I missed when I tried to use the multi-input FFmpegFrameFilter? Or the "amix" filter cannot be used yet?