diff --git a/Core/Scattering/IFormFactorBorn.h b/Core/Scattering/IFormFactorBorn.h
index fa2555168156b4afac7ec18c38ed4fe82d9aba82..2f62695ccaadeab845332cbce04075c6f2e95d3a 100644
--- a/Core/Scattering/IFormFactorBorn.h
+++ b/Core/Scattering/IFormFactorBorn.h
@@ -19,7 +19,7 @@
 #include "Core/Shapes/IShape.h"
 #include "Core/Vector/Vectors3D.h"
 
-struct SlicingEffects;
+struct SlicingEffects; // defined below
 
 //! Pure virtual base class for Born form factors.
 //!