How to give permission to specific users to list item level using PowerShell

I have tried using the below to add specific user to items but it keep getting error. For now I'm just trying to add permission to specific item but I would be changing the code to look at a column of a people picker type with multiple users.

I would appreciate if someone can tell me what I'm doing wrong

Exception calling "Add" with "1" argument(s): "" At +
$CMRSItems.RoleAssignments.Add($assignment)

$web.AllowUnsafeUpdates = $true
$user = $web.EnsureUser("domain\username")

$role = $web.RoleDefinitions["Read"]
$CMRSItems = $list.Items | where {$_['ID'] -eq 2}


$CMRSItems | ForEach-Object {

    Write-Host "ItemID: " $_['ID']
    $assignment = New-Object Microsoft.SharePoint.SPRoleAssignment($user)
    $assignment.RoleDefinitionBindings.Add($role)
    #$web.RoleAssignments.Add($assignment);
    $CMRSItems.RoleAssignments.Add($assignment)
    $CMRSItems.Update()
    $web.Dispose()
    }