diff --git a/Core/src/Material/MaterialSlab.cpp b/Core/src/Material/MaterialSlab.cpp
index a7ec8c98f4f..73d81133486 100644
--- a/Core/src/Material/MaterialSlab.cpp
+++ b/Core/src/Material/MaterialSlab.cpp
@@ -12,6 +12,7 @@
 
 #include <limits>
 #include <ostream>
+#include <stdexcept>
 
 static constexpr auto eps = 2 * std::numeric_limits<float>::epsilon();
 
@@ -22,6 +23,9 @@ Acts::MaterialSlab::MaterialSlab(const Material& material, float thickness)
       m_thickness(thickness),
       m_thicknessInX0((eps < material.X0()) ? (thickness / material.X0()) : 0),
       m_thicknessInL0((eps < material.L0()) ? (thickness / material.L0()) : 0) {
+  if (thickness < 0) {
+    throw std::runtime_error("thickness < 0");
+  }
 }
 
 Acts::MaterialSlab::MaterialSlab(const std::vector<MaterialSlab>& layers)
@@ -40,6 +44,10 @@ Acts::MaterialSlab::MaterialSlab(const std::vector<MaterialSlab>& layers)
 }
 
 void Acts::MaterialSlab::scaleThickness(float scale) {
+  if (scale < 0) {
+    throw std::runtime_error("scale < 0");
+  }
+
   m_thickness *= scale;
   m_thicknessInX0 *= scale;
   m_thicknessInL0 *= scale;
diff --git a/Fatras/include/ActsFatras/Kernel/detail/SimulationActor.hpp b/Fatras/include/ActsFatras/Kernel/detail/SimulationActor.hpp
index 32bf3f9d8ec..0c67db667f9 100644
--- a/Fatras/include/ActsFatras/Kernel/detail/SimulationActor.hpp
+++ b/Fatras/include/ActsFatras/Kernel/detail/SimulationActor.hpp
@@ -165,7 +165,8 @@ struct SimulationActor {
           // particle direction is normalized, not sure about surface normal
           auto cosIncidenceInv =
               normal.norm() / normal.dot(before.unitDirection());
-          slab.scaleThickness(cosIncidenceInv);
+          // apply abs in case `normal` and `before` produce an angle > 90°
+          slab.scaleThickness(std::abs(cosIncidenceInv));
           // run the interaction simulation
           interact(slab, result);
         }