You need add a step to make sure the assigned to value is not null before firing off the email.
You can find more details on this here. TechNet How to Send E-Mail to Specific Assigned To
So looking at SP 2010, it appears that this might be a known limitation. While there are workarounds, they are either 3rd party tools or require custom development of the workflow in order to loop through the list and send the email individually to the members of said list (Name Assigned). So unfortunately, you would need to build a script to look at the list, determine who is listed, do the mailto, then proceed to the next person. It is not something you can do straight OOTB. Alternatively, you could set the field to BCC as opposed to To, so that no one could see the other people receiving the same email from the OOTB solution.