diff --git a/doc/python/magnum.shaders.rst b/doc/python/magnum.shaders.rst
index 02270b3..41f4b49 100644
--- a/doc/python/magnum.shaders.rst
+++ b/doc/python/magnum.shaders.rst
@@ -37,3 +37,10 @@
     :data POSITION: Vertex position
     :data NORMAL: Normal direction
     :data TEXTURE_COORDINATES: 2D texture coordinates
+
+.. py:property:: magnum.shaders.Phong.alpha_mask
+    :raise AttributeError: If the shader was not created with `Flags.ALPHA_MASK`
+.. py:property:: magnum.shaders.Phong.light_positions
+    :raise ValueError: If list length is different from `light_count`
+.. py:property:: magnum.shaders.Phong.light_colors
+    :raise ValueError: If list length is different from `light_count`
diff --git a/src/python/magnum/shaders.cpp b/src/python/magnum/shaders.cpp
index 71aa9a0..4aa09f3 100644
--- a/src/python/magnum/shaders.cpp
+++ b/src/python/magnum/shaders.cpp
@@ -112,8 +112,14 @@ void shaders(py::module& m) {
             // TODO: textures, once exposed
             .def_property("shininess", nullptr,
                 &Shaders::Phong::setShininess, "Shininess")
-            .def_property("alpha_mask", nullptr,
-                &Shaders::Phong::setAlphaMask, "Alpha mask")
+            .def_property("alpha_mask", nullptr, [](Shaders::Phong& self, Float mask) {
+                if(!(self.flags() & Shaders::Phong::Flag::AlphaMask)) {
+                    PyErr_SetString(PyExc_AttributeError, "the shader was not created with alpha mask enabled");
+                    throw py::error_already_set{};
+                }
+
+                self.setAlphaMask(mask);
+            }, "Alpha mask")
             .def_property("transformation_matrix", nullptr,
                 &Shaders::Phong::setTransformationMatrix, "Set transformation matrix")
             .def_property("normal_matrix", nullptr,
@@ -121,9 +127,19 @@ void shaders(py::module& m) {
             .def_property("projection_matrix", nullptr,
                 &Shaders::Phong::setProjectionMatrix, "Set projection matrix")
             .def_property("light_positions", nullptr, [](Shaders::Phong& self, const std::vector<Vector3>& positions) {
+                if(positions.size() != self.lightCount()) {
+                    PyErr_Format(PyExc_ValueError, "expected %u items but got %u", self.lightCount(), UnsignedInt(positions.size()));
+                    throw py::error_already_set{};
+                }
+
                 self.setLightPositions(positions);
             }, "Light positions")
             .def_property("light_colors", nullptr, [](Shaders::Phong& self, const std::vector<Color4>& colors) {
+                if(colors.size() != self.lightCount()) {
+                    PyErr_Format(PyExc_ValueError, "expected %u items but got %u", self.lightCount(), UnsignedInt(colors.size()));
+                    throw py::error_already_set{};
+                }
+
                 self.setLightColors(colors);
             }, "Light colors");
     }
diff --git a/src/python/magnum/test/test_shaders_gl.py b/src/python/magnum/test/test_shaders_gl.py
index 1bd2aaf..6cd2d39 100644
--- a/src/python/magnum/test/test_shaders_gl.py
+++ b/src/python/magnum/test/test_shaders_gl.py
@@ -58,9 +58,19 @@ class Phong(GLTestCase):
         self.assertEqual(c.light_count, 3)
 
     def test_uniforms(self):
-        a = shaders.Phong()
+        a = shaders.Phong(shaders.Phong.Flags.ALPHA_MASK, 2)
         a.diffuse_color = (0.5, 1.0, 0.9)
         a.transformation_matrix = Matrix4.translation(Vector3.x_axis())
         a.projection_matrix = Matrix4.zero_init()
-        a.light_positions = [(0.5, 1.0, 0.3)]
-        a.light_colors = [Color4()]
+        a.light_positions = [(0.5, 1.0, 0.3), Vector3()]
+        a.light_colors = [Color4(), Color4()]
+        a.alpha_mask = 0.3
+
+    def test_uniforms_errors(self):
+        a = shaders.Phong()
+        with self.assertRaisesRegex(AttributeError, "the shader was not created with alpha mask enabled"):
+            a.alpha_mask = 0.3
+        with self.assertRaisesRegex(ValueError, "expected 1 items but got 0"):
+            a.light_positions = []
+        with self.assertRaisesRegex(ValueError, "expected 1 items but got 0"):
+            a.light_colors = []