c# - Trying to set a nullable relationship in Entity Framework results in compile time error? -

so need task model need set user relationship allow null values:

public class task {     public task()     {         this.id = guid.newguid();     }     public guid id { get; set; }          public virtual user personresponsible { get; set; } } 

if try set user nullable this:

public class task {     public task()     {         this.id = guid.newguid();     }     public guid id { get; set; }          public virtual user? personresponsible { get; set; } } 

i compile time error:

the type 'models.user' must non-nullable value type in order use parameter 't' in generic type or method 'system.nullable'

i've tried googling don't understand of fixes, nor of questions related entity framework. ideas?