Sending Email through a static group causes error in SageCRM CRM 7.3 SP2.1

Hi,

When I send an email to any static group I get the following error: Error processing GetValueFromField for field pers_lastname. List index out of bounds (569)

Does anyone know what is the root cause of this error?
The only thing I can think of it the field pers_lastname has a character that it cannot handle.

Thoughts?

Thanks,

Stephane