diff --git a/include/util.h b/include/util.h
index 8cd140e08d9288930fc70245b36fa4bb2b5d529a..6a3e057b463e77e0163a05b562d0ed27dabb4dd1 100644
--- a/include/util.h
+++ b/include/util.h
@@ -80,7 +80,7 @@ namespace util {
   //=====================================================================================
   inline auto p_dummy(const std::vector<eic::ReconstructedParticle>& parts)
   {
-    std::vector<ROOT::Math::PxPyPzMVector> momenta{tracks.size()};
+    std::vector<ROOT::Math::PxPyPzMVector> momenta{parts.size()};
     // transform our raw tracker info into proper 4-momenta
     std::transform(parts.begin(), parts.end(), momenta.begin(), [](const auto& parts) {
       const double px = parts.p.x;