diff --git a/hnn_core/tests/test_dipole.py b/hnn_core/tests/test_dipole.py
index 538f34ff9..f3ea26cbd 100644
--- a/hnn_core/tests/test_dipole.py
+++ b/hnn_core/tests/test_dipole.py
@@ -45,6 +45,8 @@ def test_dipole(tmpdir, run_hnn_core_fixture):
     # Test wrong argument to plot_dipole()
     with pytest.raises(TypeError, match='dpl must be an instance of'):
         plot_dipole([dipole, 10], show=False)
+    with pytest.raises(AttributeError, match="object has no attribute 'append"):
+        plot_dipole(np.array([dipole, dipole]), average=True, show=False)
 
     # Test IO
     dipole.write(dpl_out_fname)
diff --git a/hnn_core/viz.py b/hnn_core/viz.py
index 9c04c4fc9..87fde29d6 100644
--- a/hnn_core/viz.py
+++ b/hnn_core/viz.py
@@ -274,7 +274,7 @@ def plot_dipole(dpl, tmin=None, tmax=None, ax=None, layer='agg', decim=None,
         _validate_type(this_dpl, Dipole, 'dpl', 'Dipole, list of Dipole')
 
     if average:
-        dpl = dpl + [average_dipoles(dpl)]
+        dpl.append(average_dipoles(dpl))
 
     scale_applied = dpl[0].scale_applied