diff --git a/surface_analyses/patches.py b/surface_analyses/patches.py
index 3b0c176..f77c2e7 100644
--- a/surface_analyses/patches.py
+++ b/surface_analyses/patches.py
@@ -18,6 +18,8 @@ def find_patches(faces, should_be_in_patch):
     patches : list of np.ndarray objects.
         Each array contains a list of vertex indices that define a patch.
     """
+    faces = np.asarray(faces)
+    should_be_in_patch = np.asarray(should_be_in_patch)
     assert len(should_be_in_patch.shape) == 1
     assert len(faces.shape) == 2 and faces.shape[1] == 3
     n_vertices = should_be_in_patch.shape[0]
diff --git a/test/patches_test.py b/test/patches_test.py
new file mode 100644
index 0000000..f5a1f96
--- /dev/null
+++ b/test/patches_test.py
@@ -0,0 +1,35 @@
+import pytest
+import numpy as np
+
+import surface_analyses.surface as surf
+import surface_analyses.patches as patches
+
+@pytest.fixture
+def minimal():
+    vertices = [[0., 0., 0.], [0., 0., 1.], [0., 1., 1.], [0., 2., 1.], [0., 2., 2.]]
+    faces = [[0, 1, 2], [2, 3, 4]]
+    s = surf.Surface(vertices, faces)
+    s.data['DATA'] = [-2, -1, 0, 1, 2]
+    return s
+
+def test_finds_patches_full(minimal):
+    p = patches.find_patches(minimal.faces, [True, True, True, True, True])
+    assert len(p) == 1
+    assert np.all(p[0] == [0, 1, 2, 3, 4])
+
+def test_finds_patches_separate(minimal):
+    p = patches.find_patches(minimal.faces, [True, True, False, True, True])
+    assert len(p) == 2
+    assert np.all(p[0] == [0, 1])
+    assert np.all(p[1] == [3, 4])
+    area = minimal.vertex_areas()
+    assert area[p[0]].sum() == 1.0
+
+def test_finds_patches_separate(minimal):
+    p = patches.find_patches(minimal.faces, [True, True, False, True, True])
+    assert len(p) == 2
+    assert np.all(p[0] == [0, 1])
+    assert np.all(p[1] == [3, 4])
+    area = minimal.vertex_areas()
+    assert area[p[0]].sum() == pytest.approx(1/3)
+    assert area[p[1]].sum() == pytest.approx(1/3)
diff --git a/test/surface_test.py b/test/surface_test.py
index 50ea511..ba6f8ba 100644
--- a/test/surface_test.py
+++ b/test/surface_test.py
@@ -41,3 +41,7 @@ def test_ses_gisttools():
     out = surf.compute_ses_gisttools(grid, [[0.0, 0.0, 0.0]], [1.0])
     # area of a unit sphere is 4*pi = 12.566, but we are not really exact
     assert np.isclose(out.areas().sum(), 12.5876)
+
+def test_triangle_area(minimal):
+    areas = minimal.vertex_areas()
+    assert np.allclose(areas, [1/6, 1/6, 1/6])