Unable to Pass variable value to For each loop

Hello Everyone,

I'm new to Poweshell, Someone tell what's wrong in below code? i want to pass $server value to for each loop but below code is failing

#getServerList
$Servers = Invoke-Sqlcmd -ServerInstance "localhost" -Query "SELECT [server_name] FROM [DBName].[dbo].[ServersList]" -Database "DBName"

#Write-output $server
foreach($Server in $Servers)
{
#SQl Query
$sql = "SELECT @@SERVERNAME AS 'ServerName', DB_NAME(dbid) AS 'Database',name, CONVERT(BIGINT, size) * 8 AS 'size_in_kb', filename
FROM master..sysaltfiles"
Invoke-Sqlcmd -ServerInstance $server -query $sql -Database master -OutputAs DataTables |
#WriteData
Write-SqlTableData -ServerInstance localhost -Database master -SchemaName dbo -TableName DatabasesSizes -Force
}