Odd formatting with PowerShell and Json

PowerShell and Json

Doesn't work:
$data = "`"{'message':{'body':'{0} - {1}'}}`"" -f $message, $url

Throws:

Error formatting a string: Input string was not in a correct format..
At Script.ps1:11 char:50
+ $data = "`"{'message':{'body':'{0} - {1}'}}`"" -f <<<<  $message, $url
    + CategoryInfo          : InvalidOperation: (System.Object[]:Object[]) [], RuntimeException
    + FullyQualifiedErrorId : FormatError

Works:

$data = "`"{'message':{'body':'" + $message + " - " + $url + "'}}`""