diff --git a/Core/Algorithms/MultiLayerRoughnessDWBASimulation.cpp b/Core/Algorithms/MultiLayerRoughnessDWBASimulation.cpp
index 0aaab51ad48eefec349dddaec6efc8294282ade4..54b040824350e4264f3332f270136f9955837be1 100644
--- a/Core/Algorithms/MultiLayerRoughnessDWBASimulation.cpp
+++ b/Core/Algorithms/MultiLayerRoughnessDWBASimulation.cpp
@@ -74,6 +74,7 @@ void MultiLayerRoughnessDWBASimulation::runProtected()
 
 double MultiLayerRoughnessDWBASimulation::evaluate(const SimulationElement& sim_element)
 {
+    if (sim_element.getAlphaMean()<0.0) return 0.0;
     kvector_t q = sim_element.getMeanQ();
     double wavelength = sim_element.getWavelength();
     double autocorr(0.0);