EF code-first, cannot delete foreign key relation

I have a two models shown here:

public class Application
{
  public string Name { get; set; }
  public virtual ICollection<ApplicationTransaction> ApplicationTransactions { get; set; }
}

and

public class ApplicationTransaction
{
  public long ApplicationId { get; set; }
  public virtual Application Application { get; set; }
}

I tried to delete all ApplicationTransaction of Application with this code:

var app = _repository.Get<Application>(i => i.Id == 1);
app.ApplicationTransactions.Clear();
Context.SaveChanges();

but when context goes to save the changes, an error occurs:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.