Huh.  'b' is not a valid bech32 alphabet character.

Verify you are doing something like: 

$ ./target/release/relay-tester ws://localhost:8080/ nsec19eg50yp0qt3ulwqm9l5p5jh2y5ezw742erc2r82kxa3r0ys33meqj9fmdx  nsec18f2kapsh77ju22vx3hl647ufwuv0jh4twm6jng4fmzxge35cfklqv8zujq

The code changed recently to need two nsecs.