"Invalid String Format" error message received when attempting to link customer to Salesforce account.

This error message is typically caused by unlinking and relinking a Fusebill customer to a Salesforce Customer Account in a faster pace than the system has time to finalize the changes. 

The error message indicates that the system is looking for a blank field, but the field already has a value in it. The field in question is the FusebillData.ID that is on the customer.

To fix this, manually remove the value in the FusebillData.ID field and link the customer again.