Error sending Email from workflow

I've got a Sharepoint list inside a project server site. When a user creates an item in this list I want to send an Email to another user.

I've been able to get the email address using REST and store it in a workflow variable (as a string). But when I use the action "send email" the workflow gets cancelled and I get this error:

System.ArgumentNullException: Value cannot be null. Parameter name: Input at Microsoft.Activities.Hosting.Runtime.Subroutine.SubroutineChild.Execute(CodeActivityContext

The weird thing here is that I do a log action of the variable value right before the email action, and it has a correct email address.

Is there any other field that could fire this error? Is there any problem trying to send an email to an address stored in a string variable? May it be another thing?

Any help would be appreciated :)