Setting a custom lookup field with PHP SDK

Setting a custom lookup field with PHP SDK

I have a custom module "investments" that has a lookup field to the contacts module.
If I get an instance of ZCRMRecord with the correct entity id etc can I just set this relationship using something like

$contact value is returned from a call to searchRecordsByEmail
$investment->setFieldValue("Investor"$contact);

However this throws an invalid data error.

Couldnt find any examples of the correct way to do this.