Unclear how to use `--reference` with `--prepare`

gsnedders@gsnedders-margot ~ % hyperfine --show-output -N -r1 -L delay 0.2,0.4,0.6 -p 'sleep {delay}' 'echo a'
Benchmark 1: echo a (delay = 0.2)
a
  Time (abs ≡):          9.2 ms               [User: 1.7 ms, System: 3.1 ms]
 
Benchmark 2: echo a (delay = 0.4)
a
  Time (abs ≡):          9.1 ms               [User: 1.9 ms, System: 3.5 ms]
 
Benchmark 3: echo a (delay = 0.6)
a
  Time (abs ≡):         10.3 ms               [User: 1.8 ms, System: 3.5 ms]
 
Summary
  echo a (delay = 0.4) ran
    1.01 times faster than echo a (delay = 0.2)
    1.14 times faster than echo a (delay = 0.6)
gsnedders@gsnedders-margot ~ % hyperfine --show-output -N -r1 -L delay 0.2,0.4,0.6 -p 'sleep {delay}' --reference 'echo a (delay = 0.4)' 'echo a'
Benchmark 1: echo a (delay = 0.4)
sleep: invalid time interval: {delay}
usage: sleep number[unit] [...]
Unit can be 's' (seconds, the default), m (minutes), h (hours), or d (days).
Error: The preparation command terminated with a non-zero exit code. Append ' || true' to the command if you are sure that this can be ignored.
gsnedders@gsnedders-margot ~ % hyperfine --show-output -N -r1 -L delay 0.2,0.4,0.6 -p 'sleep {delay}' --reference 'delay = 0.4' 'echo a'
Benchmark 1: delay = 0.4
sleep: invalid time interval: {delay}
usage: sleep number[unit] [...]
Unit can be 's' (seconds, the default), m (minutes), h (hours), or d (days).
Error: The preparation command terminated with a non-zero exit code. Append ' || true' to the command if you are sure that this can be ignored.
gsnedders@gsnedders-margot ~ % hyperfine --show-output -N -r1 -L delay 0.2,0.4,0.6 -p 'sleep {delay}' --reference 'echo b' 'echo a'
Benchmark 1: echo b
sleep: invalid time interval: {delay}
usage: sleep number[unit] [...]
Unit can be 's' (seconds, the default), m (minutes), h (hours), or d (days).
Error: The preparation command terminated with a non-zero exit code. Append ' || true' to the command if you are sure that this can be ignored.