diff --git a/kwant/physics/tests/test_noise.py b/kwant/physics/tests/test_noise.py
index e10d845d5f6ad63aeefcfa9fc6b0fe2fe688d488..a8bcb27f569885b178a173436e0f54f3383884a0 100644
--- a/kwant/physics/tests/test_noise.py
+++ b/kwant/physics/tests/test_noise.py
@@ -39,6 +39,16 @@ def test_twoterminal_input():
     assert_raises(ValueError, two_terminal_shotnoise, sol)
 
 
+class LeadWithOnlySelfEnergy(object):
+    def __init__(self, lead):
+        self.lead = lead
+
+    def self_energy(self, energy, args=(), kwargs={}):
+        assert args == ()
+        assert kwargs == {}
+        return self.lead.self_energy(energy)
+
+
 def test_twoterminal():
     """Shot noise in a two-terminal conductor"""
 
@@ -52,13 +62,6 @@ def test_twoterminal():
     assert_almost_equal(noise_should_be, two_terminal_shotnoise(sol))
 
     # replace leads successively with self-energy
-    class LeadWithOnlySelfEnergy(object):
-        def __init__(self, lead):
-            self.lead = lead
-
-        def self_energy(self, energy):
-            return self.lead.self_energy(energy)
-
     fsys.leads[0] = LeadWithOnlySelfEnergy(fsys.leads[0])
     sol = kwant.solve(fsys)
     assert_almost_equal(noise_should_be, two_terminal_shotnoise(sol))
diff --git a/kwant/solvers/common.py b/kwant/solvers/common.py
index 5323b299836b57a8afebda3c9b8a53bf6595104f..1c4d42f3b12c899c05402974e35e96c8f21ee477 100644
--- a/kwant/solvers/common.py
+++ b/kwant/solvers/common.py
@@ -237,7 +237,7 @@ class SparseSolver(object):
                     # See comment about zero-shaped sparse matrices at the top.
                     rhs.append(np.zeros((lhs.shape[1], 0)))
             else:
-                sigma = lead.self_energy(energy, args=args, kwargs=kwargs)
+                sigma = lead.self_energy(energy, args, kwargs)
                 lead_info.append(sigma)
                 indices = np.r_[tuple(slice(offsets[i], offsets[i + 1])
                                       for i in interface)]
diff --git a/kwant/solvers/tests/_test_sparse.py b/kwant/solvers/tests/_test_sparse.py
index 95c8bbfb541c8e561ba3cde0aee9f604c10011eb..c6b10d8f69ea95524e49774f34e8159cd30487f8 100644
--- a/kwant/solvers/tests/_test_sparse.py
+++ b/kwant/solvers/tests/_test_sparse.py
@@ -17,6 +17,16 @@ chain = kwant.lattice.chain()
 sq = square = kwant.lattice.square()
 
 
+class LeadWithOnlySelfEnergy(object):
+    def __init__(self, lead):
+        self.lead = lead
+
+    def self_energy(self, energy, args=(), kwargs={}):
+        assert args == ()
+        assert kwargs == {}
+        return self.lead.self_energy(energy)
+
+
 # Test output sanity: that an error is raised if no output is requested,
 # and that solving for a subblock of a scattering matrix is the same as taking
 # a subblock of the full scattering matrix.
@@ -264,13 +274,6 @@ def test_tricky_singular_hopping(solve):
 # Test equivalence between self-energy and scattering matrix representations.
 # Also check that transmission works.
 def test_self_energy(solve):
-    class LeadWithOnlySelfEnergy(object):
-        def __init__(self, lead):
-            self.lead = lead
-
-        def self_energy(self, energy):
-            return self.lead.self_energy(energy)
-
     np.random.seed(4)
     system = kwant.Builder()
     left_lead = kwant.Builder(kwant.TranslationalSymmetry((-1,)))
@@ -314,13 +317,6 @@ def test_self_energy(solve):
 
 
 def test_self_energy_reflection(solve):
-    class LeadWithOnlySelfEnergy(object):
-        def __init__(self, lead):
-            self.lead = lead
-
-        def self_energy(self, energy):
-            return self.lead.self_energy(energy)
-
     np.random.seed(4)
     system = kwant.Builder()
     left_lead = kwant.Builder(kwant.TranslationalSymmetry((-1,)))
diff --git a/kwant/system.py b/kwant/system.py
index 50e1eaf08441ce81578cc774d21a3797bdf50825..44d7ad142f32120c1c93f5d8606071542598664b 100644
--- a/kwant/system.py
+++ b/kwant/system.py
@@ -69,7 +69,7 @@ class FiniteSystem(System):
     ------------------
     leads : sequence of lead objects
         Each lead object has to provide a method
-        ``self_energy(energy, *args, **kwargs)``.
+        ``self_energy(energy, args, kwargs)``.
     lead_interfaces : sequence of sequences of integers
         Each sub-sequence contains the indices of the system sites to which the
         lead is connected.