Error: Cannot convert value "Note" to type "System.Type". Error: "Invalid cast from 'Microsoft.SharePoint.SPFieldType' to 'System.Type'."

Hi,

I am trying to access a multiline text field using powershell. I am tring to use "GetFieldValueAsText" option.

I have tried the below code:

 foreach($item in $listItems)
    {
        Write-Host $item.Title


        $calculatedfield = $item.Fields["Test Notes"] -as [Microsoft.SharePoint.SPFieldType]::Note;

        $item["Test Notes"] = $calculatedfield.GetFieldValueAsText($item["Test Notes"]);


        $item.SystemUpdate();

    }

I am getting the below error:

Cannot convert value "Note" to type "System.Type". Error: "Invalid cast from 'Microsoft.SharePoint.SPFieldType' to 'System.Type'."
At \RemoveCharacters.ps1:26 char:9+         $calculatedfield = $item.Fields["Test Notes"] -as [Microsoft. ...+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : InvalidArgument: (:) [], RuntimeException+ FullyQualifiedErrorId : InvalidCastIConvertible

You cannot call a method on a null-valued expression.
At \RemoveCharacters.ps1:28 char:9
+         $item["Test Notes"] = $calculatedfield.GetFieldValueAsText($i ...+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : InvalidOperation: (:) [], RuntimeException+ FullyQualifiedErrorId : InvokeMethodOnNull

How to fix this? How to use the option GetFieldValueAsText in powershell for multiline column?

Thanks