Error when creating custom permissions using powershell

I'm trying to create custom permissions in my Sharepoint site via Powershell using the following script:

$spSite = Get-SPSite $url
$spWeb = $spSite | Get-SPWeb

foreach($option in $configuration.Config.permissions.permission)
{
    if($spWeb.RoleDefinitions[$option.name] -eq $null)
    {
        $spRoleDefinition = New-Object Microsoft.SharePoint.SPRoleDefinition
        $spRoleDefinition.Name = $option.name
        $spRoleDefinition.Description = $option.description

        $spRoleDefinition.BasePermissions = $option.bases
        $spWeb.RoleDefinitions.Add($spRoleDefinition)
    }
}
$spWeb.Dispose()
$spSite.Dispose()

And I keep receiving the following error:

Exception calling "Add" with "1" argument(s): "Collection was of a fixed size."
At C:\xxxx\xxxxxx\xxxxx:70 char:4
+             $spWeb.RoleDefinitions.Add($spRoleDefinition)
+             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : NotSupportedException

Thanks for the help.