The answer is:
Nora tells Helmer that there are no letters in the box, while the audience knows that there is a letter from Krogstad.
Dramatic irony occurs when the audience possesses information that the characters do not.
In Henrik Ibsen's "A Doll's House," Helmer does not know about Krogstad's letter in the mailbox, which reveals Nora's forgery to borrow money. The tension increases when Helmer wishes to check the mailbox and Nora intends to prevent him to do so by calling his attention.