diff --git a/offline/packages/calovtxreco/CaloVtxAlgoCNN.cc b/offline/packages/calovtxreco/CaloVtxAlgoCNN.cc
index df1a18f54d..e95ab5314f 100644
--- a/offline/packages/calovtxreco/CaloVtxAlgoCNN.cc
+++ b/offline/packages/calovtxreco/CaloVtxAlgoCNN.cc
@@ -27,7 +27,7 @@ namespace
   const std::string GeomNodeIhc = "TOWERGEOM_HCALIN";
 }  // namespace
 
-// onnxruntime session (pImpl, keeps Ort types out of the header)
+// onnxruntime session (pImpl)
 struct CaloVtxAlgoCNN::OnnxSession
 {
   Ort::Env env{ORT_LOGGING_LEVEL_WARNING, "CaloVtxAlgoCNN"};
@@ -44,6 +44,7 @@ struct CaloVtxAlgoCNN::OnnxSession
   }
 };
 
+CaloVtxAlgoCNN::CaloVtxAlgoCNN() = default;
 CaloVtxAlgoCNN::~CaloVtxAlgoCNN() = default;
 
 int CaloVtxAlgoCNN::Init(PHCompositeNode * /*topNode*/)
diff --git a/offline/packages/calovtxreco/CaloVtxAlgoCNN.h b/offline/packages/calovtxreco/CaloVtxAlgoCNN.h
index c1770de5b3..64b0dc2f8e 100644
--- a/offline/packages/calovtxreco/CaloVtxAlgoCNN.h
+++ b/offline/packages/calovtxreco/CaloVtxAlgoCNN.h
@@ -25,8 +25,8 @@ class CaloVtxAlgoCNN : public CaloVtxAlgo
     kOHC = 2
   };
 
-  explicit CaloVtxAlgoCNN() = default;
-  ~CaloVtxAlgoCNN() override; 
+  CaloVtxAlgoCNN();            // defined in the .cc, where OnnxSession is complete
+  ~CaloVtxAlgoCNN() override;  // (required for the unique_ptr pImpl member)
 
   int Init(PHCompositeNode *topNode) override;
   int CalculateVertex(PHCompositeNode *topNode, float &zvtx) override;