diff --git a/src/libslic3r/Geometry.cpp b/src/libslic3r/Geometry.cpp
index 25db3cce0..7bab0ed7a 100644
--- a/src/libslic3r/Geometry.cpp
+++ b/src/libslic3r/Geometry.cpp
@@ -877,8 +877,7 @@ double rotation_diff_z(const Vec3d &rot_xyz_from, const Vec3d &rot_xyz_to)
     const Eigen::AngleAxisd angle_axis(rotation_xyz_diff(rot_xyz_from, rot_xyz_to));
     const Vec3d& axis  = angle_axis.axis();
     const double angle = angle_axis.angle();
-#if 0
-//#ifndef NDEBUG
+#ifndef NDEBUG
     if (std::abs(angle) > 1e-8) {
         assert(std::abs(axis.x()) < 1e-8);
         assert(std::abs(axis.y()) < 1e-8);
@@ -887,4 +886,15 @@ double rotation_diff_z(const Vec3d &rot_xyz_from, const Vec3d &rot_xyz_to)
     return (axis.z() < 0) ? -angle : angle;
 }
 
+double rotation_diff_z(const Transform3d &trafo_from, const Transform3d &trafo_to)
+{
+    auto  m  = trafo_to.linear() * trafo_from.linear().inverse();
+    assert(std::abs(m.determinant() - 1));
+    Vec3d vx = m * Vec3d(1., 0., 0);
+    // Verify that the linear part of rotation from trafo_from to trafo_to rotates around Z and is unity.
+    assert(std::abs(std::hypot(vx.x(), vx.y()) - 1.) < 1e-5);
+    assert(std::abs(vx.z()) < 1e-5);
+    return atan2(vx.y(), vx.x());
+}
+
 }} // namespace Slic3r::Geometry
diff --git a/src/libslic3r/Geometry.hpp b/src/libslic3r/Geometry.hpp
index acaad6af3..d7cf32611 100644
--- a/src/libslic3r/Geometry.hpp
+++ b/src/libslic3r/Geometry.hpp
@@ -547,6 +547,7 @@ extern Eigen::Quaterniond rotation_xyz_diff(const Vec3d &rot_xyz_from, const Vec
 // Rotation by Z to align rot_xyz_from to rot_xyz_to.
 // This should only be called if it is known, that the two rotations only differ in rotation around the Z axis.
 extern double rotation_diff_z(const Vec3d &rot_xyz_from, const Vec3d &rot_xyz_to);
+extern double rotation_diff_z(const Transform3d &trafo_from, const Transform3d &trafo_to);
 
 // Is the angle close to a multiple of 90 degrees?
 inline bool is_rotation_ninety_degrees(double a)
diff --git a/src/libslic3r/Print.cpp b/src/libslic3r/Print.cpp
index e7305234f..eff0bcd45 100644
--- a/src/libslic3r/Print.cpp
+++ b/src/libslic3r/Print.cpp
@@ -404,7 +404,7 @@ bool Print::sequential_print_horizontal_clearance_valid(const Print& print, Poly
       }
 	    // Make a copy, so it may be rotated for instances.
 	    Polygon convex_hull0 = it_convex_hull->second;
-		const double z_diff = Geometry::rotation_diff_z(model_instance0->get_rotation(), print_object->instances().front().model_instance->get_rotation());
+		const double z_diff = Geometry::rotation_diff_z(model_instance0->get_matrix(), print_object->instances().front().model_instance->get_matrix());
 		if (std::abs(z_diff) > EPSILON)
 			convex_hull0.rotate(z_diff);
 	    // Now we check that no instance of convex_hull intersects any of the previously checked object instances.
diff --git a/src/libslic3r/SLAPrint.cpp b/src/libslic3r/SLAPrint.cpp
index d4023f64d..dc5e2b519 100644
--- a/src/libslic3r/SLAPrint.cpp
+++ b/src/libslic3r/SLAPrint.cpp
@@ -190,14 +190,13 @@ static std::vector<SLAPrintObject::Instance> sla_instances(const ModelObject &mo
     std::vector<SLAPrintObject::Instance> instances;
     assert(! model_object.instances.empty());
     if (! model_object.instances.empty()) {
-        Vec3d rotation0 = model_object.instances.front()->get_rotation();
-        rotation0(2) = 0.;
+        const Transform3d& trafo0 = model_object.instances.front()->get_matrix();
         for (ModelInstance *model_instance : model_object.instances)
             if (model_instance->is_printable()) {
                 instances.emplace_back(
                     model_instance->id(),
                     Point::new_scale(model_instance->get_offset(X), model_instance->get_offset(Y)),
-                    float(Geometry::rotation_diff_z(rotation0, model_instance->get_rotation())));
+                    float(Geometry::rotation_diff_z(trafo0, model_instance->get_matrix())));
             }
     }
     return instances;