Failed to find recording

[nekoshang]

Using OBS 29 with obs websocket 4.9.1 compat.
Although I can start and stop recording OBS from within Reaper, on stopping I get this error:

Failed to find recording. do you have the right EXTENSION configured in the source code for this action? 
extension: mkv  

However my videos are being recorded in the matroska container with mkv extension from within OBS.

Perhaps there is some compatibility issue with obs/obs-websockets? Or I have not set things up properly?

Thanks for any help.