Powershell error. WriteToServer. column does not allow DBNull.Value - sql-server

Question

I am using following powershell script to collect server storage information from multiple servers and into the one of the SQL server table.
I am getting an error while executing following powershell script
Error:-
Exception calling "WriteToServer" with "1" argument(s): "Column
'server_name' does not allow DBNull.Value.
Table has only one column with all server names in it. Powershell script grabs server name from database table and collect server storage information. It works for few servers however it fails for few server with above error.
Table
CREATE TABLE [dbo].[server_space_lku](
[server_name] [varchar](50) NOT NULL,
CONSTRAINT [PK_server_lku] PRIMARY KEY CLUSTERED)
I am using following powershell script. How can I fix the issue?
param($destServer, $destDb)
function Get-SqlData
{
param([string]$serverName=$(throw 'serverName is required.'), [string]$databaseName=$(throw 'databaseName is required.'),
[string]$query=$(throw 'query is required.'))
Write-Verbose "Get-SqlData serverName:$serverName databaseName:$databaseName query:$query"
$connString = "Server=$serverName;Database=$databaseName;Integrated Security=SSPI;"
$da = New-Object "System.Data.SqlClient.SqlDataAdapter" ($query,$connString)
$dt = New-Object "System.Data.DataTable"
[void]$da.fill($dt)
$dt
} #Get-SqlData
function Get-Vol
{
param($computerName)
Get-WmiObject -computername "$ComputerName" Win32_Volume -filter "DriveType=3 AND SystemVolume = $false" |
foreach { add-member -in $_ -membertype noteproperty UsageDT $((Get-Date).ToString("yyyy-MM-dd"))
add-member -in $_ -membertype noteproperty SizeGB $([math]::round(($_.Capacity/1GB),2))
add-member -in $_ -membertype noteproperty FreeGB $([math]::round(($_.FreeSpace/1GB),2))
add-member -in $_ -membertype noteproperty PercentFree $([math]::round((([float]$_.FreeSpace/[float]$_.Capacity) * 100),2)) -passThru} |
select UsageDT, SystemName, Name, Label, SizeGB, FreeGB, PercentFree
}# Get-Vol
function Out-DataTable
{
param($Properties="*")
Begin
{
$dt = new-object Data.datatable
$First = $true
}
Process
{
$DR = $DT.NewRow()
foreach ($item in $_ | Get-Member -type *Property $Properties ) {
$name = $item.Name
if ($first) {
$Col = new-object Data.DataColumn
$Col.ColumnName = $name
$DT.Columns.Add($Col)
}
$DR.Item($name) = $_.$name
}
$DT.Rows.Add($DR)
$First = $false
}
End
{
return #(,($dt))
}
}# Out-DataTable
function Write-DataTableToDatabase
{
param($destServer,$destDb,$destTbl,$dt)
$connectionString = "Data Source=$destServer;Integrated Security=true;Initial Catalog=$destdb;"
$bulkCopy = new-object ("Data.SqlClient.SqlBulkCopy") $connectionString
$bulkCopy.DestinationTableName = "$destTbl"
$bulkCopy.WriteToServer($dt)
}# Write-DataTableToDatabase
Get-SqlData $destServer $destDb "SELECT server_name FROM server_space_lku" |
foreach {
#Get just the server name portion if instance name is included
$computerName = $_.server_name -replace "\\.*|,.*"
$dt = Get-Vol $computerName | Out-DataTable
Write-DataTableToDatabase $destServer $destDb 'vol_space' $dt
}