diff --git a/src/iguana/algorithms/clas12/ZVertexFilter/Algorithm.cc b/src/iguana/algorithms/clas12/ZVertexFilter/Algorithm.cc
index d63dcc96..1ff9712f 100644
--- a/src/iguana/algorithms/clas12/ZVertexFilter/Algorithm.cc
+++ b/src/iguana/algorithms/clas12/ZVertexFilter/Algorithm.cc
@@ -46,6 +46,7 @@ namespace iguana::clas12 {
   }
 
   concurrent_key_t ZVertexFilter::PrepareEvent(int const runnum, concurrent_key_t key) const {
+    m_log->Trace("calling PrepareEvent({}, {})", runnum, key);
     if(o_runnum->NeedsHashing()) {
       std::hash<int> hash_ftn;
       auto hash_key = hash_ftn(runnum);
@@ -53,13 +54,14 @@ namespace iguana::clas12 {
         Reload(runnum, hash_key);
       return hash_key;
     }
-    if(o_runnum->Load(key) != runnum)
+    if(o_runnum->IsEmpty() || o_runnum->Load(key) != runnum)
       Reload(runnum, key);
     return key;
   }
 
   void ZVertexFilter::Reload(int const runnum, concurrent_key_t key) const {
     std::lock_guard<std::mutex> const lock(m_mutex); // NOTE: be sure to lock successive `ConcurrentParam::Save` calls !!!
+    m_log->Trace("-> calling Reload({}, {})", runnum, key);
     o_runnum->Save(runnum, key);
     o_zcuts->Save(GetOptionVector<double>("cuts", {GetConfig()->InRange("runs", runnum), "cuts"}), key);
     // FIXME: handle PIDs
diff --git a/src/iguana/services/ConcurrentParam.cc b/src/iguana/services/ConcurrentParam.cc
index 4674447b..de2349e2 100644
--- a/src/iguana/services/ConcurrentParam.cc
+++ b/src/iguana/services/ConcurrentParam.cc
@@ -64,6 +64,7 @@ namespace iguana {
   template <typename T>
   void SingleThreadParam<T>::Save(T const& value, concurrent_key_t const key)
   {
+    this->m_empty = false;
     m_value = value;
   }
 
@@ -71,12 +72,14 @@ namespace iguana {
   void MemoizedParam<T>::Save(T const& value, concurrent_key_t const key)
   {
     std::lock_guard<std::mutex> const lock(this->m_mutex);
+    this->m_empty = false;
     m_container.insert({key, value});
   }
 
   template <typename T>
   void ThreadPoolParam<T>::Save(T const& value, concurrent_key_t const key)
   {
+    this->m_empty = false;
     throw std::runtime_error("TODO: 'threadpool' model not yet implemented");
   }
 
@@ -102,4 +105,24 @@ namespace iguana {
     throw std::runtime_error("TODO: 'threadpool' model not yet implemented");
   }
 
+  // ==================================================================================
+  // GetSize() methods
+  // ==================================================================================
+  template <typename T>
+  std::size_t SingleThreadParam<T>::GetSize() const
+  {
+    return 1;
+  }
+
+  template <typename T>
+  std::size_t MemoizedParam<T>::GetSize() const
+  {
+    return m_container.size();
+  }
+
+  template <typename T>
+  std::size_t ThreadPoolParam<T>::GetSize() const
+  {
+    throw std::runtime_error("TODO: 'threadpool' model not yet implemented");
+  }
 }
diff --git a/src/iguana/services/ConcurrentParam.h b/src/iguana/services/ConcurrentParam.h
index a1653be3..1185d818 100644
--- a/src/iguana/services/ConcurrentParam.h
+++ b/src/iguana/services/ConcurrentParam.h
@@ -40,6 +40,12 @@ namespace iguana {
       /// @returns true if hashing is needed
       bool NeedsHashing() const { return m_needs_hashing; }
 
+      /// @returns the size of the internal data storage container
+      virtual std::size_t GetSize() const = 0;
+
+      /// @returns true if no value has been saved
+      bool IsEmpty() const { return m_empty; }
+
     protected:
 
       /// whether this `ConcurrentParam` needs hashing for calling `::Load` or `::Save`
@@ -48,6 +54,9 @@ namespace iguana {
       /// mutex
       std::mutex m_mutex;
 
+      /// whether this `ConcurrentParam` has something saved
+      bool m_empty{true};
+
   };
 
   // ==================================================================================
@@ -64,6 +73,7 @@ namespace iguana {
       T const Load(concurrent_key_t const key = 0) const override;
       void Save(T const& value, concurrent_key_t const key = 0) override;
       bool HasKey(concurrent_key_t const key) const override;
+      std::size_t GetSize() const override;
 
     private:
       T m_value;
@@ -86,6 +96,7 @@ namespace iguana {
       T const Load(concurrent_key_t const key = 0) const override;
       void Save(T const& value, concurrent_key_t const key = 0) override;
       bool HasKey(concurrent_key_t const key) const override;
+      std::size_t GetSize() const override;
 
     private:
       container_t m_container;
@@ -109,6 +120,7 @@ namespace iguana {
       T const Load(concurrent_key_t const key = 0) const override;
       void Save(T const& value, concurrent_key_t const key = 0) override;
       bool HasKey(concurrent_key_t const key) const override;
+      std::size_t GetSize() const override;
 
     private:
       container_t m_container;