Bug with "cbc-pillowfight" - the parameter "-p" (--key-prefix) doesn't work

cbc-pillowfight -U couchbase://localhost/test --username Administrator --password password --min-size 1024   --max-size 1024   --num-items 10000 --num-threads 1 --set-pct 40 --batch-size 200  --json --timings --key-prefix myprefix

Output:
[86400.000007] Running. Press Ctrl-C to terminate... libc++abi: terminating with uncaught exception of type std::invalid_argument: stol: no conversion Abort trap: 6

When removing the "--key-prefix" argument  - it works fine.

It looks like a bug in libcouchbase 3. (it used to work fine with libcouchbase 2.x)