diff --git a/Generators/TruthUtils/TruthUtils/MagicNumbers.h b/Generators/TruthUtils/TruthUtils/MagicNumbers.h
index b81f4f962c3e2e9062d95c54d93e96e80cab56b1..e619454826c922d593821acc28a688d5b7dee818 100644
--- a/Generators/TruthUtils/TruthUtils/MagicNumbers.h
+++ b/Generators/TruthUtils/TruthUtils/MagicNumbers.h
@@ -46,6 +46,8 @@ constexpr int FORWARDTRANSPORTMODELSTATUS = 212;
 /// @brief This barcode is used by objects matched to particles from pile-up interactions in standard MC Production
 constexpr int SUPPRESSED_PILEUP_BARCODE(std::numeric_limits<int32_t>::max());
 
+constexpr int UNDEFINED_ID = 0;
+
 constexpr int INVALID_PARTICLE_BARCODE = -1;
 
 constexpr int SINGLE_PARTICLE = 10001;