diff --git a/corsika/modules/sibyll/InteractionModel.hpp b/corsika/modules/sibyll/InteractionModel.hpp
index 16b71369627e832e2e195eb3f0355a39511423b9..58b9d36e1b0ade04d423957f5c13e463feb7eb7b 100644
--- a/corsika/modules/sibyll/InteractionModel.hpp
+++ b/corsika/modules/sibyll/InteractionModel.hpp
@@ -15,6 +15,8 @@
 
 namespace corsika::sibyll {
 
+  class FourMomentum; // fwd-decl
+
   /**
    * @brief sibyll::InteractionModel provides the SIBYLL proton-nucleus interaction model.
    *