fillet between two perpendicular line?

I want to insert a fillet between two (initially) perpendicular lines.
I tried using the tweak command–>fillet—>provide fillet radius and curve IDs (the two lines), but it didn’t work. It gives me an error:
ERROR:
Encountered an uncaught exception 'Attempted get of empty DLIList
'. Command failed.

Any suggestion?

Could you provide a journal file or model that shows the curves?