diff --git a/src/shapes/disk.cpp b/src/shapes/disk.cpp
index 7d575ebc..922f8e83 100644
--- a/src/shapes/disk.cpp
+++ b/src/shapes/disk.cpp
@@ -105,7 +105,7 @@ public:
 		Vector dpdu = trafo(Vector(1, 0, 0));
 		Vector dpdv = trafo(Vector(0, 1, 0));
 
-		if (std::abs(dot(dpdu, dpdv)) > 1e-3f)
+		if (std::abs(dot(normalize(dpdu), normalize(dpdv))) > 1e-3f)
 			Log(EError, "Error: 'toWorld' transformation contains shear!");
 
 		if (std::abs(dpdu.length() / dpdv.length() - 1) > 1e-3f)
diff --git a/src/shapes/rectangle.cpp b/src/shapes/rectangle.cpp
index 2be94648..694d136d 100644
--- a/src/shapes/rectangle.cpp
+++ b/src/shapes/rectangle.cpp
@@ -105,7 +105,7 @@ public:
 		m_frame = Frame(normalize(m_dpdu), normalize(m_dpdv), normal);
 
 		m_invSurfaceArea = 1.0f / getSurfaceArea();
-		if (std::abs(dot(m_dpdu, m_dpdv)) > Epsilon)
+		if (std::abs(dot(normalize(m_dpdu), normalize(m_dpdv))) > Epsilon)
 			Log(EError, "Error: 'toWorld' transformation contains shear!");
 	}