SharePoint 2013 Workflow Permission "Value does not fall within the expected range"

Hi,

I'm using SharePoint 2013 Workflow to assign List item permission to users using the REST Service.

First, I break the inheritance of the item using  
http://<Sharepoint Site>/_api/Web/lists/GetByTitle('List1')/items(1)/breakroleinheritance(copyRoleAssignment=true)

Second, assigning permissions to User4 using
http://<Sharepoint_site>/_api/Web/lists/GetByTitle('List1')/items(1)/roleassignments/addroleassignment(principalid=21,roleDefId=1073741827)
an error "Value does not fall within the expected range." is thrown. 

In http://<Sharepint_Site>/_api/Web/lists/GetByTitle('List1')/items(1)/roleassignment, only "Everyone", "User1", "User2" and "User3" are being inherited. How do I assign permissions to other users that are not inherited (E.g. User4, User5)? 

Thanks in advance!