diff --git a/src/integrators/path/volpath.cpp b/src/integrators/path/volpath.cpp
index ec85df18..690e8012 100644
--- a/src/integrators/path/volpath.cpp
+++ b/src/integrators/path/volpath.cpp
@@ -378,14 +378,16 @@ public:
 		while (true) {
 			surface = scene->rayIntersect(ray, *its);
 
-			if (surface && (interactions == maxInteractions ||
-				!(its->getBSDF()->getType() & BSDF::ENull)))
-				/* Encountered an occluder -- zero transmittance. */
-				break;
-
 			if (medium)
 				transmittance *= medium->evalTransmittance(Ray(ray, 0, its->t), sampler);
 
+			if (surface && (interactions == maxInteractions ||
+				!(its->getBSDF()->getType() & BSDF::ENull) ||
+				its->isEmitter())) {
+				/* Encountered an occluder / light source */
+				break;
+			}
+
 			if (!surface)
 				break;
 
diff --git a/src/librender/shape.cpp b/src/librender/shape.cpp
index 6d173a4a..9e8865f8 100644
--- a/src/librender/shape.cpp
+++ b/src/librender/shape.cpp
@@ -156,13 +156,13 @@ void Shape::addChild(const std::string &name, ConfigurableObject *child) {
 		m_subsurface = static_cast<Subsurface *>(child);
 	} else if (cClass->derivesFrom(MTS_CLASS(Medium))) {
 		if (name == "interior") {
-			Assert(m_interiorMedium == NULL);
+			Assert(m_interiorMedium == NULL || m_interiorMedium == child);
 			if (m_subsurface != NULL)
 				Log(EError, "Shape \"%s\" has both an interior medium "
 					"and a subsurface scattering model -- please choose one or the other!", getName().c_str());
 			m_interiorMedium = static_cast<Medium *>(child);
 		} else if (name == "exterior") {
-			Assert(m_exteriorMedium == NULL);
+			Assert(m_exteriorMedium == NULL || m_exteriorMedium == child);
 			m_exteriorMedium = static_cast<Medium *>(child);
 			if (m_emitter)
 				m_emitter->setMedium(m_exteriorMedium);