The current jmap -histo command can accept "parallel=<N>" options. But if "jmap -histo:parallel"
is used without any value specified, it prints “Fail: invalid option: 'parallel‘”.

From the specification, it states how "parallel=" is used:
  =======================================
histo-options:
      live count only live objects (takes precedence if both "live" and "all" are specified)
      all count all objects in the heap (default if one of "live" or "all" is not specified)
      file=<file> dump data to <file>
      parallel=<number> parallel threads number for heap iteration:
                                  parallel=0 default behavior, use predefined number of threads
                                  parallel=1 disable parallel heap iteration
                                  parallel=<N> use N threads for parallel heap iteration
  ========================================

But does not mention about using "parallel" without any value.

Using default behavior (same as parallel=0) is better for "parallel" rather than printing an error message.