"Android Flash Tool" fails for sunfish userdebug builds

[Jacob S]

Hi , am am trying to flash a pixel4a device with a aosp_sunfish-userdebug build using Android Flash Tool latest master build  - 6983044  ,  ( also tried with building master, and some other latest android 11 branches ) and after flashing  -  Android Flash Tool reports success , but device always boots to fastboot with the error:

no valid slot to boot

Is this a known issue with sunfish ?

how can I successfully flash the device with userdebug image ( I don't require any specific version just needs to be with test-keys ) ?