Removing permissions from a sharepoint folder in a for loop

I am trying to remove all the permissions from folders in sharepoint using a powershell script but only some of the permissions are being removed. When I manually run the powershell commands I can see that there are four role assignmebers but only two are removed.

PS C:\support\scripts> $rvn.Item.RoleAssignments.Count
4
PS C:\support\scripts> for($i=0;$i -lt $rvn.Item.RoleAssignments.Count; $i++){
>> $ra = $rvn.Item.RoleAssignments[$i]
>> write-host "$i $($ra.Member.LoginName)"
>> }
>>
0 Microsoft CRM Visitors
1 rmvd\timc
2 rmvd\amyw
3 rmvd\AaronR
PS C:\support\scripts> for($i=0;$i -lt $rvn.Item.RoleAssignments.Count; $i++){
>> $ra = $rvn.Item.RoleAssignments[$i]
>> write-host "$i $($ra.Member.LoginName)"
>> $rvn.Item.RoleAssignments.Remove($ra.Member)
>> }
>>
0 Microsoft CRM Visitors
1 rmvd\amyw
PS C:\support\scripts>