Inconsistent Write-DataTable behavior when writing to a SQL Server table

I'm trying to create a script that imports data from multiple database servers into one central data repository using the Write-DataTable function (source code here). The script I came up with works if the target table (RPRMBSDEVDB81.dba_rep..DBBakSizes) is empty, however, if there is data in the table it blows up.

What I'm doing:

1. I delete everything from target table via SQL Server Mgmt Studio.
2. I run the PowerShell script and it runs successfully and data ends up in the table.
3. I delete data via SSMS for one server from the target table.
4. I run the PowerShell script again, verifying I have unique data in my data table, and it errors out (see complete error below my script).

My script:

#Clear screen (for testing)
Clear-Host

#SQLCMD timeout parameter
$QueryTimeout = 120

#Get list of servers to import data from
$sql_serverlist = "
SELECT sl.HostName
    ,sl.ServerName + '.' + sl.Domain AS ServerName
    ,ISNULL(MAX(bs.ReportDate),'1/1/1980') AS ReportDate
FROM dbo.ServerList sl
    LEFT OUTER JOIN dbo.DBBakSizes bs ON bs.ServerName = sl.HostName
WHERE sl.Import = 1
AND sl.Active = 1
GROUP BY sl.HostName
    ,sl.ServerName + '.' + sl.Domain;
"
$servers = Invoke-Sqlcmd -ServerInstance RPRMBSDEVDB81 -Database dba_rep -Query $sql_serverlist

#Define path to Write-DataTable module
$Location = "C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn\Modules\Write-DataTable"
#Load Write-DataTable module
Import-Module $Location\Write-DataTable.psm1

#Setup DataTable
$dt = New-Object Data.DataTable
$col1 = New-Object Data.DataColumn ServerName,([string])
$col2 = New-Object Data.DataColumn DatabaseName,([string])
$col3 = New-Object Data.DataColumn BackupType,([string])
$col4 = New-Object Data.DataColumn UsedCompression,([int])
$col5 = New-Object Data.DataColumn UsedChecksum,([int])
$col6 = New-Object Data.DataColumn MostRecentFull_Date,([datetime])
$col7 = New-Object Data.DataColumn MostRecentFull_Sec,([int])
$col8 = New-Object Data.DataColumn MostRecentFull_MB,([int])
$col9 = New-Object Data.DataColumn MostRecentOther,([string])
$col10 = New-Object Data.DataColumn MostRecentOther_Date,([datetime])
$col11 = New-Object Data.DataColumn MostRecentOther_Sec,([int])
$col12 = New-Object Data.DataColumn MostRecentOther_MB,([int])
$col13 = New-Object Data.DataColumn ReportDate,([datetime])
$dt.columns.add($col1)
$dt.columns.add($col2)
$dt.columns.add($col3)
$dt.columns.add($col4)
$dt.columns.add($col5)
$dt.columns.add($col6)
$dt.columns.add($col7)
$dt.columns.add($col8)
$dt.columns.add($col9)
$dt.columns.add($col10)
$dt.columns.add($col11)
$dt.columns.add($col12)
$dt.columns.add($col13)

#Loop through servers and pull in bak file data
foreach ($server in $servers)
{
    #Retrieve ServerName and MAX(ReportDate) from array
    $hostname = $server[0]
    $servername = $server[1]
    $reportdate = $server[2].ToString()

    #Build SQL to retrieve records for import
    $sql_bakdata = "
    SELECT  '$hostname' AS ServerName
        ,DatabaseName
        ,BackupType
        ,UsedCompression
        ,UsedChecksum
        ,MostRecentFull_Date
        ,MostRecentFull_Sec
        ,MostRecentFull_MB
        ,MostRecentOther
        ,MostRecentOther_Date
        ,MostRecentOther_Sec
        ,MostRecentOther_MB
        ,ReportDate
    FROM rp_util.dbo.DBBakSizes
    WHERE CAST(ReportDate AS SMALLDATETIME) > '$reportdate';
    "

    #Run SQL and capture results in array
    $dt += Invoke-Sqlcmd -ServerInstance $servername -Query $sql_bakdata -QueryTimeout $QueryTimeout
}

#Load data
Write-DataTable -ServerInstance "RPRMBSDEVDB81" -Database "dba_rep" -TableName "DBBakSizes" -Data $dt

The error:

Write-DataTable : System.Management.Automation.MethodInvocationException:
Exception calling "WriteToServer" with "1" argument(s): "Object reference not
set to an instance of an object." ---> System.NullReferenceException: Object
reference not set to an instance of an object.
   at System.Data.SqlClient.SqlBulkCopy.WriteToServer(DataRow[] rows)
   at WriteToServer(Object , Object[] )
   at System.Management.Automation.DotNetAdapter.AuxiliaryMethodInvoke(Object target, Object[] arguments, MethodInformation methodInformation, Object[] originalArguments)
   --- End of inner exception stack trace ---
   at System.Management.Automation.DotNetAdapter.AuxiliaryMethodInvoke(Object target, Object[] arguments, MethodInformation methodInformation, Object[] originalArguments)
   at System.Management.Automation.ParserOps.CallMethod(Token token, Object target, String methodName, Object[] paramArray, Boolean callStatic, Object valueToSet)
   at System.Management.Automation.MethodCallNode.InvokeMethod(Object target, Object[] arguments, Object value)
   at System.Management.Automation.MethodCallNode.Execute(Array input, Pipe outputPipe, ExecutionContext context)
   at System.Management.Automation.ParseTreeNode.Execute(Array input, Pipe outputPipe, ArrayList& resultList, ExecutionContext context)
   at System.Management.Automation.StatementListNode.ExecuteStatement(ParseTreeNode statement, Array input, Pipe outputPipe, ArrayList& resultList, ExecutionContext context).Message
At line:2 char:16
+ Write-DataTable <<<<  -ServerInstance "RPRMBSDEVDB81" -Database "dba_rep" -TableName "DBBakSizes" -Data $dt
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,Write-DataTable