Environment- MS Exchange 2010

We have about 10 Retention Policies in our environment and I would like to get a count of the number of mailboxes assigned to each policy.  Here is the script I thought should do the trick, but didn't.

get-mailbox -resultsize unlimited| group-object -property:RetentionPolicy| sort-object Count -desc| select RetentionPolicy, Count

I have also tried -

get-mailbox -resultsize unlimited| group-object -property RetentionPolicy| sort-object Count -desc| select RetentionPolicy, Count

As I have seen this syntax as well.  I have also tried running these scripts as Administrator to make sure that wasn't the issue.

While the count column gets populated the retention policy column is simply blank for all policies.  When I remove the group-object section the script does populate the retention policy column, but ungrouped.  The only thing I can think of is that the group-object doesn't like grouping by retention policy, though why this would be I can't fathom.

Can anyone straighten out this puzzle?

TIA

Have you tried selecting the properties first then group them?

get-mailbox -resultsize unlimited | select RetentionPolicy, Count | group-object -property RetentionPolicy | sort-object Count -desc

Thanks Clayman2.  This worked.  

Lesson learned - if it doesn't work one way shake things up and see what happens then.

In case it helps someone in the future here is the final script for counting and grouping mailboxes and retention policies.

get-mailbox -resultsize unlimited| select RetentionPolicy, count| group-object -property RetentionPolicy| sort-object count -desc| ft Name, Count