diff --git a/src/integrators/mega_path.cpp b/src/integrators/mega_path.cpp
index 18b4d98d..d820fc96 100644
--- a/src/integrators/mega_path.cpp
+++ b/src/integrators/mega_path.cpp
@@ -57,6 +57,7 @@ class MegakernelPathTracingInstance final : public ProgressiveIntegrator::Instan
         auto swl = spectrum->sample(spectrum->node()->is_fixed() ? 0.f : sampler()->generate_1d());
         SampledSpectrum beta{swl.dimension(), camera_weight};
         SampledSpectrum Li{swl.dimension()};
+        Float eta_scale = def(1.f);
 
         auto ray = camera_ray;
         auto pdf_bsdf = def(1e16f);
@@ -109,7 +110,6 @@ class MegakernelPathTracingInstance final : public ProgressiveIntegrator::Instan
 
             // evaluate material
             auto surface_tag = it->shape().surface_tag();
-            auto eta_scale = def(1.f);
 
             $outline {
                 PolymorphicCall<Surface::Closure> call;
@@ -137,8 +137,8 @@ class MegakernelPathTracingInstance final : public ProgressiveIntegrator::Instan
                     // apply eta scale
                     auto eta = closure->eta().value_or(1.f);
                     $switch(surface_sample.event) {
-                        $case(Surface::event_enter) { eta_scale = sqr(eta); };
-                        $case(Surface::event_exit) { eta_scale = sqr(1.f / eta); };
+                        $case(Surface::event_enter) { eta_scale *= sqr(eta); };
+                        $case(Surface::event_exit) { eta_scale *= sqr(1.f / eta); };
                     };
                 });
             };
diff --git a/src/util/frame.cpp b/src/util/frame.cpp
index 81091f93..ab37aa3f 100644
--- a/src/util/frame.cpp
+++ b/src/util/frame.cpp
@@ -19,7 +19,7 @@ Frame::Frame() noexcept
       _n{make_float3(0.f, 0.f, 1.f)} {}
 
 Frame Frame::make(Expr<float3> n) noexcept {
-    auto sgn = sign(n.z);
+    auto sgn = copysign(1.f, n.z);
     auto a = -1.f / (sgn + n.z);
     auto b = n.x * n.y * a;
     auto s = make_float3(1.f + sgn * sqr(n.x) * a, sgn * b, -sgn * n.x);