Set target audience field

I am struggling to set the target audience filed on a list.

I have a list containing the following columns

EmployeeNumber      | Targets                              |

001                            | AD group 1, AD Group 2    |

I have another list of items that I want to set the target audience.

I have enabled the audience targeting feature on the list which added the target audience filed.  In the workflow I can pickup the AD group id's from the first list doing a lookup but when I try to use the set field value workflow action the target audience list is not updated in the target list.

Help :-)