pgrep returns 0 when incorrect command line syntax used

my belief is that return code 0 should indicate successful match

from man page:
snip
EXIT STATUS
       0 One or more processes matched the criteria.
       1 No processes matched.
       2 Syntax error in the command line.
       3 Fatal error: out of memory etc.
snip

dgarley@TPX61:~$ pgrep cat
2537
2541
dgarley@TPX61:~$ echo $?
0
dgarley@TPX61:~$ # COMMENT notice the return code 0 above, there was a successful match
dgarley@TPX61:~$ pgrep -Z cat
pgrep: invalid option -- 'Z'

Usage:
 pgrep [options] <pattern>

Options:
 -d, --delimiter <string> specify output delimiter
 -l, --list-name list PID and process name
 -v, --inverse negates the matching
 -w, --lightweight list all TID
 -c, --count count of matching processes
 -f, --full use full process name to match
 -g, --pgroup <id,...> match listed process group IDs
 -G, --group <gid,...> match real group IDs
 -n, --newest select most recently started
 -o, --oldest select least recently started
 -P, --parent <ppid,...> match only child processes of the given parent
 -s, --session <sid,...> match session IDs
 -t, --terminal <tty,...> match by controlling terminal
 -u, --euid <id,...> match by effective IDs
 -U, --uid <id,...> match by real IDs
 -x, --exact match exactly with the command name
 -F, --pidfile <file> read PIDs from file
 -L, --logpidfile fail if PID file is not locked
 --ns <pid> match the processes that belong to the same
                           namespace as <pid>
 --nslist <ns,...> list which namespaces will be considered for
                           the --ns option.
                           Available namespaces: ipc, mnt, net, pid, user, uts

 -h, --help display this help and exit
 -V, --version output version information and exit

For more details see pgrep(1).
dgarley@TPX61:~$ echo $?
0
dgarley@TPX61:~$ # COMMENT notice return code 0 above even though no match (Intentionally used invalid command switch -Z)
dgarley@TPX61:~$

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: procps 1:3.3.9-1ubuntu2.2
ProcVersionSignature: Ubuntu 3.13.0-61.100-generic 3.13.11-ckt22
Uname: Linux 3.13.0-61-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.15
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Oct 1 16:44:54 2015
InstallationDate: Installed on 2014-12-02 (303 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
SourcePackage: procps
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.sysctl.conf: 2015-04-28T09:14:08.035779