diff --git a/kwant/system.py b/kwant/system.py
index 1d6fd10693a87f7f036dff14fb23f42e84ccfe43..925117881531eb2cba14654965c57300b526e801 100644
--- a/kwant/system.py
+++ b/kwant/system.py
@@ -148,8 +148,9 @@ class InfiniteSystem(System):
     def self_energy(self, energy, args=()):
         """Return self-energy of a lead.
 
-        The returned matrix has the shape (n, n), where n is
-        ``sum(self.num_orbitals(i) for i in range(self.slice_size))``.
+        The returned matrix has the shape (s, s), where s is
+        ``sum(self.num_orbitals(i)
+              for i in range(self.graph.num_nodes - self.slice_size))``.
         """
         ham = self.slice_hamiltonian(args=args)
         shape = ham.shape