Cannot find encoding

Hi,

I am trying to get a SOAP request working. At this line in my code:

$info = $soapClient->__call("getLegalUnit", array($ap_param));

I get this error:

PHP Fatal error:  SOAP-ERROR: Encoding: Cannot find encoding in <file> 
<line no>.

What does "encoding" mean in this context, and how do I specify it?

Best regards,

Finn